WriteBack Throttle kill the performace of the disk

[Nicheal]

Hi,

I'm currently finding that enable WritebackThrottle lead to lower IOPS
for large number of small io. Since WritebackThrottle calls
fdatasync(fd) to flush an object content to disk, large number of
ramdom small io always cause the WritebackThrottle to submit one or
two 4k io every time.
Thus, it is much slower than the global sync in
FileStore::sync_entry(). Note:: here, I use xfs as the FileStore
underlying filesystem. So I would know that if any impact when I
disable Writeback throttles. I cannot catch the idea on the website
(http://ceph.com/docs/master/dev/osd_internals/wbthrottle/).
Large number of inode will cause longer time to sync, but submitting a
batch of write to disk always faster than submitting few io update to
the disk.

Nicheal
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[Mark Nelson]

Post by Nicheal
Hi,
I'm currently finding that enable WritebackThrottle lead to lower IOPS
for large number of small io. Since WritebackThrottle calls
fdatasync(fd) to flush an object content to disk, large number of
ramdom small io always cause the WritebackThrottle to submit one or
two 4k io every time.
Thus, it is much slower than the global sync in
FileStore::sync_entry(). Note:: here, I use xfs as the FileStore
underlying filesystem. So I would know that if any impact when I
disable Writeback throttles. I cannot catch the idea on the website
(http://ceph.com/docs/master/dev/osd_internals/wbthrottle/).
Large number of inode will cause longer time to sync, but submitting a
batch of write to disk always faster than submitting few io update to
the disk.

Hi Nichael,

When the wbthrottle code was introduced back around dumpling we had to
increase the sync intervals quite a bit to get it performing similarly
to cuttlefish. Have you tried playing with the various wbthrottle xfs
tuneables to see if you can improve the behaviour?

OPTION(filestore_wbthrottle_enable, OPT_BOOL, true)
OPTION(filestore_wbthrottle_xfs_bytes_start_flusher, OPT_U64, 41943040)
OPTION(filestore_wbthrottle_xfs_bytes_hard_limit, OPT_U64, 419430400)
OPTION(filestore_wbthrottle_xfs_ios_start_flusher, OPT_U64, 500)
OPTION(filestore_wbthrottle_xfs_ios_hard_limit, OPT_U64, 5000)
OPTION(filestore_wbthrottle_xfs_inodes_start_flusher, OPT_U64, 500)

Mark

Post by Nicheal
Nicheal
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[Gregory Farnum]

Post by Mark Nelson

Post by Nicheal
Hi,
I'm currently finding that enable WritebackThrottle lead to lower IOPS
for large number of small io. Since WritebackThrottle calls
fdatasync(fd) to flush an object content to disk, large number of
ramdom small io always cause the WritebackThrottle to submit one or
two 4k io every time.
Thus, it is much slower than the global sync in
FileStore::sync_entry(). Note:: here, I use xfs as the FileStore
underlying filesystem. So I would know that if any impact when I
disable Writeback throttles. I cannot catch the idea on the website
(http://ceph.com/docs/master/dev/osd_internals/wbthrottle/).
Large number of inode will cause longer time to sync, but submitting a
batch of write to disk always faster than submitting few io update to
the disk.

Hi Nichael,
When the wbthrottle code was introduced back around dumpling we had to
increase the sync intervals quite a bit to get it performing similarly to
cuttlefish. Have you tried playing with the various wbthrottle xfs
tuneables to see if you can improve the behaviour?
OPTION(filestore_wbthrottle_enable, OPT_BOOL, true)
OPTION(filestore_wbthrottle_xfs_bytes_start_flusher, OPT_U64, 41943040)
OPTION(filestore_wbthrottle_xfs_bytes_hard_limit, OPT_U64, 419430400)
OPTION(filestore_wbthrottle_xfs_ios_start_flusher, OPT_U64, 500)
OPTION(filestore_wbthrottle_xfs_ios_hard_limit, OPT_U64, 5000)
OPTION(filestore_wbthrottle_xfs_inodes_start_flusher, OPT_U64, 500)

In particular, these are semi-tuned for a standard spinning hard
drive. If you have an SSD as your backing store, you'll want to put
them all way up.
Alternatively, if you have a very large journal, you will see the
flusher as slowing down shorter benchmarks, because it's trying to
keep the journal from getting too far ahead of the backing store. But
this is deliberate; it's making you pay a closer approximation to the
true cost up front instead of letting you overload the system and then
have all your writes get very slow as syncfs calls start taking tens
of seconds.
-Greg
Software Engineer #42 @ http://inktank.com | http://ceph.com
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[Nicheal]

Yes, Greg.
But Unix based system always have a parameter dirty_ratio to prevent
the system memory from being exhausted. If Journal speed is so fast
while backing store cannot catch up with Journal, then the backing
store write will be blocked by the hard limitation of system dirty
pages. The problem here may be that system call, sync(), cannot return
since the system always has lots of dirty pages. Consequently, 1)
FileStore::sync_entry() will be timeout and then ceph_osd_daemon
abort. 2) Even if the thread is not timed out, Since the Journal
committed point cannot be updated so that the Journal will be blocked,
waiting for the sync() return and update Journal committed point.
So the Throttle is added to solve the above problems, right?
However, in my tested ARM ceph cluster(3nodes, 9osds, 3osds/node), it
will cause problem (SSD as journal, and HDD as data disk, fio 4k
ramdom write iodepth 64):
WritebackThrottle enable: Based on blktrace, we trace the back-end
hdd io behaviour. Because of frequently calling fdatasync() in
Writeback Throttle, it cause every back-end hdd spent more time to
finish one io. This causes the total sync time longer. For example,
default sync_max_interval is 5 seconds, total dirty data in 5 seconds
is 10M. If I disable WritebackThrottle, 10M dirty data will be sync to
disk within 4 second, So cat /proc/meminfo, the dirty data of my
system is always clean(near zero). However, If I enable
WritebackThrottle, fdatasync() slows down the sync process. Thus, it
seems 8-9M random io will be sync to the disk within 5s. Thus the
dirty data is always growing to the critical point (system
up-limitation), and then sync_entry() is always timed out. So I means,
in my case, disabling WritebackThrottle, I may always have 600 IOPS.
If enabling WritebackThrottle, IOPS always drop to 200 since fdatasync
cause back-end HDD disk overloaded.
So I would like that we can dynamically throttle the IOPS in
FileStore. We cannot know the average sync() speed of the back-end
Store since different disk own different IO performance. However, we
can trace the average write speed in FileStore and Journal, Also, we
can know, whether start_sync() is return and finished. Thus, If this
time, Journal is writing so fast that the back-end cannot catch up the
Journal(e.g. 1000IOPS/s). We cannot Throttle the Journal speed(e.g.
800IOPS/s) in next operation interval(the interval maybe 1 to 5
seconds, in the third second, Thottle become 1000*e^-x where x is the
tick interval, ), if in this interval, Journal write reach the
limitation, the following submitting write should waiting in OSD
waiting queue.So in this way, Journal may provide a boosting IO, but
finally, back-end sync() will return and catch up with Journal become
we always slow down the Journal speed after several seconds.
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[Mark Nelson]

Post by Nicheal
Yes, Greg.
But Unix based system always have a parameter dirty_ratio to prevent
the system memory from being exhausted. If Journal speed is so fast
while backing store cannot catch up with Journal, then the backing
store write will be blocked by the hard limitation of system dirty
pages. The problem here may be that system call, sync(), cannot return
since the system always has lots of dirty pages. Consequently, 1)
FileStore::sync_entry() will be timeout and then ceph_osd_daemon
abort. 2) Even if the thread is not timed out, Since the Journal
committed point cannot be updated so that the Journal will be blocked,
waiting for the sync() return and update Journal committed point.
So the Throttle is added to solve the above problems, right?

Greg or Sam can correct me if I'm wrong, but I always thought of the
wbthrottle code as being more an attempt to smooth out spikes in write
throughput to prevent the journal from getting too far ahead of the
backing store. IE have more frequent, shorter flush periods rather than
less frequent longer ones. For Ceph that is's probably a reasonable
idea since you want all of the OSDs behaving as consistently as possible
to prevent hitting the max outstanding client IOs/Bytes on the client
and starving other ready OSDs. I'm not sure it's worked out in practice
as well as it might have in theory, though I'm not sure we've really
investigated what's going on enough to be sure.

Post by Nicheal
However, in my tested ARM ceph cluster(3nodes, 9osds, 3osds/node), it
will cause problem (SSD as journal, and HDD as data disk, fio 4k
WritebackThrottle enable: Based on blktrace, we trace the back-end
hdd io behaviour. Because of frequently calling fdatasync() in
Writeback Throttle, it cause every back-end hdd spent more time to
finish one io. This causes the total sync time longer. For example,
default sync_max_interval is 5 seconds, total dirty data in 5 seconds
is 10M. If I disable WritebackThrottle, 10M dirty data will be sync to
disk within 4 second, So cat /proc/meminfo, the dirty data of my
system is always clean(near zero). However, If I enable
WritebackThrottle, fdatasync() slows down the sync process. Thus, it
seems 8-9M random io will be sync to the disk within 5s. Thus the
dirty data is always growing to the critical point (system
up-limitation), and then sync_entry() is always timed out. So I means,
in my case, disabling WritebackThrottle, I may always have 600 IOPS.
If enabling WritebackThrottle, IOPS always drop to 200 since fdatasync
cause back-end HDD disk overloaded.

We never did a blktrace investigation, but we did see pretty bad
performance with the default wbthrottle code when it was first
implemented. We ended up raising the throttles pretty considerably in
dumpling RC2. It would be interesting to repeat this test on an Intel
system.

Post by Nicheal
So I would like that we can dynamically throttle the IOPS in
FileStore. We cannot know the average sync() speed of the back-end
Store since different disk own different IO performance. However, we
can trace the average write speed in FileStore and Journal, Also, we
can know, whether start_sync() is return and finished. Thus, If this
time, Journal is writing so fast that the back-end cannot catch up the
Journal(e.g. 1000IOPS/s). We cannot Throttle the Journal speed(e.g.
800IOPS/s) in next operation interval(the interval maybe 1 to 5
seconds, in the third second, Thottle become 1000*e^-x where x is the
tick interval, ), if in this interval, Journal write reach the
limitation, the following submitting write should waiting in OSD
waiting queue.So in this way, Journal may provide a boosting IO, but
finally, back-end sync() will return and catch up with Journal become
we always slow down the Journal speed after several seconds.

I will wait for Sam's input, but it seems reasonable to me. Perhaps you
might write it up as a blueprint for CDS?

Mark
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[Wido den Hollander]

Post by Mark Nelson

Post by Nicheal
Yes, Greg.
But Unix based system always have a parameter dirty_ratio to prevent
the system memory from being exhausted. If Journal speed is so fast
while backing store cannot catch up with Journal, then the backing
store write will be blocked by the hard limitation of system dirty
pages. The problem here may be that system call, sync(), cannot return
since the system always has lots of dirty pages. Consequently, 1)
FileStore::sync_entry() will be timeout and then ceph_osd_daemon
abort. 2) Even if the thread is not timed out, Since the Journal
committed point cannot be updated so that the Journal will be blocked,
waiting for the sync() return and update Journal committed point.
So the Throttle is added to solve the above problems, right?

Greg or Sam can correct me if I'm wrong, but I always thought of the
wbthrottle code as being more an attempt to smooth out spikes in write
throughput to prevent the journal from getting too far ahead of the
backing store. IE have more frequent, shorter flush periods rather than
less frequent longer ones. For Ceph that is's probably a reasonable
idea since you want all of the OSDs behaving as consistently as possible
to prevent hitting the max outstanding client IOs/Bytes on the client
and starving other ready OSDs. I'm not sure it's worked out in practice
as well as it might have in theory, though I'm not sure we've really
investigated what's going on enough to be sure.

I thought that as well. So in the case of a SSD-based OSD where the
journal is on a partition #1 and the data on #2 you would disable
wbthrottle, correct?

Since the journal is just as fast as the data partition.

Post by Mark Nelson

Post by Nicheal
However, in my tested ARM ceph cluster(3nodes, 9osds, 3osds/node), it
will cause problem (SSD as journal, and HDD as data disk, fio 4k
WritebackThrottle enable: Based on blktrace, we trace the back-end
hdd io behaviour. Because of frequently calling fdatasync() in
Writeback Throttle, it cause every back-end hdd spent more time to
finish one io. This causes the total sync time longer. For example,
default sync_max_interval is 5 seconds, total dirty data in 5 seconds
is 10M. If I disable WritebackThrottle, 10M dirty data will be sync to
disk within 4 second, So cat /proc/meminfo, the dirty data of my
system is always clean(near zero). However, If I enable
WritebackThrottle, fdatasync() slows down the sync process. Thus, it
seems 8-9M random io will be sync to the disk within 5s. Thus the
dirty data is always growing to the critical point (system
up-limitation), and then sync_entry() is always timed out. So I means,
in my case, disabling WritebackThrottle, I may always have 600 IOPS.
If enabling WritebackThrottle, IOPS always drop to 200 since fdatasync
cause back-end HDD disk overloaded.

We never did a blktrace investigation, but we did see pretty bad
performance with the default wbthrottle code when it was first
implemented. We ended up raising the throttles pretty considerably in
dumpling RC2. It would be interesting to repeat this test on an Intel
system.

Post by Nicheal
So I would like that we can dynamically throttle the IOPS in
FileStore. We cannot know the average sync() speed of the back-end
Store since different disk own different IO performance. However, we
can trace the average write speed in FileStore and Journal, Also, we
can know, whether start_sync() is return and finished. Thus, If this
time, Journal is writing so fast that the back-end cannot catch up the
Journal(e.g. 1000IOPS/s). We cannot Throttle the Journal speed(e.g.
800IOPS/s) in next operation interval(the interval maybe 1 to 5
seconds, in the third second, Thottle become 1000*e^-x where x is the
tick interval, ), if in this interval, Journal write reach the
limitation, the following submitting write should waiting in OSD
waiting queue.So in this way, Journal may provide a boosting IO, but
finally, back-end sync() will return and catch up with Journal become
we always slow down the Journal speed after several seconds.

I will wait for Sam's input, but it seems reasonable to me. Perhaps you
might write it up as a blueprint for CDS?
Mark
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--
Wido den Hollander
42on B.V.
Ceph trainer and consultant

Phone: [phone]
Skype: contact42on
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[Nicheal]

Post by Mark Nelson

Post by Nicheal
Yes, Greg.
But Unix based system always have a parameter dirty_ratio to prevent
the system memory from being exhausted. If Journal speed is so fast
while backing store cannot catch up with Journal, then the backing
store write will be blocked by the hard limitation of system dirty
pages. The problem here may be that system call, sync(), cannot return
since the system always has lots of dirty pages. Consequently, 1)
FileStore::sync_entry() will be timeout and then ceph_osd_daemon
abort. 2) Even if the thread is not timed out, Since the Journal
committed point cannot be updated so that the Journal will be blocked,
waiting for the sync() return and update Journal committed point.
So the Throttle is added to solve the above problems, right?

Greg or Sam can correct me if I'm wrong, but I always thought of the
wbthrottle code as being more an attempt to smooth out spikes in write
throughput to prevent the journal from getting too far ahead of the
backing store. IE have more frequent, shorter flush periods rather than
less frequent longer ones. For Ceph that is's probably a reasonable
idea since you want all of the OSDs behaving as consistently as possible
to prevent hitting the max outstanding client IOs/Bytes on the client
and starving other ready OSDs. I'm not sure it's worked out in practice
as well as it might have in theory, though I'm not sure we've really
investigated what's going on enough to be sure.
I thought that as well. So in the case of a SSD-based OSD where the
journal is on a partition #1 and the data on #2 you would disable
wbthrottle, correct?

Yes, Wido. But it also depends, I don't know you environment, but I
can provide tips here:
First of all, if you do large number of small io (e.g. 4k), the
bottleneck maybe your CPU, my xeon E3 1230 v2 can just support 2 SSD
OSD/node if I test 4k write. So disabling wbthrottle can save your
cpu cost and improve performance.
Secondly, if your cpu is not bottleneck (supposing you use a
powerful server 2*Xeon E5), then if you use SSD can provide power-loss
data protection, you can mount you SSD with nobarrier(If you don't
know the concept of filesystem writebarrier, please refer to
http://xfs.org/index.php/XFS_FAQ#Write_barrier_support) so that
fdatasync() would be quite efficient to smooth your IOPS.
If you don't care how to improve the performance based on the ceph
source code, my suggestion is that try different tuning under your
environment and chose the better one.

Post by Mark Nelson
Since the journal is just as fast as the data partition.

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[Sage Weil]

Post by Mark Nelson

Post by Nicheal
Yes, Greg.
But Unix based system always have a parameter dirty_ratio to prevent
the system memory from being exhausted. If Journal speed is so fast
while backing store cannot catch up with Journal, then the backing
store write will be blocked by the hard limitation of system dirty
pages. The problem here may be that system call, sync(), cannot return
since the system always has lots of dirty pages. Consequently, 1)
FileStore::sync_entry() will be timeout and then ceph_osd_daemon
abort. 2) Even if the thread is not timed out, Since the Journal
committed point cannot be updated so that the Journal will be blocked,
waiting for the sync() return and update Journal committed point.
So the Throttle is added to solve the above problems, right?

Greg or Sam can correct me if I'm wrong, but I always thought of the
wbthrottle code as being more an attempt to smooth out spikes in write
throughput to prevent the journal from getting too far ahead of the backing
store. IE have more frequent, shorter flush periods rather than less frequent
longer ones. For Ceph that is's probably a reasonable idea since you want all
of the OSDs behaving as consistently as possible to prevent hitting the max
outstanding client IOs/Bytes on the client and starving other ready OSDs. I'm
not sure it's worked out in practice as well as it might have in theory,
though I'm not sure we've really investigated what's going on enough to be
sure.

Right. The fdatasync strategy means that the overall throughput is lower,
but the latencies are much more consistent. Without the throttling we had
huge spikes, which is even more problematic.

Post by Mark Nelson

Post by Nicheal
However, in my tested ARM ceph cluster(3nodes, 9osds, 3osds/node), it
will cause problem (SSD as journal, and HDD as data disk, fio 4k
WritebackThrottle enable: Based on blktrace, we trace the back-end
hdd io behaviour. Because of frequently calling fdatasync() in
Writeback Throttle, it cause every back-end hdd spent more time to
finish one io. This causes the total sync time longer. For example,
default sync_max_interval is 5 seconds, total dirty data in 5 seconds
is 10M. If I disable WritebackThrottle, 10M dirty data will be sync to
disk within 4 second, So cat /proc/meminfo, the dirty data of my
system is always clean(near zero). However, If I enable
WritebackThrottle, fdatasync() slows down the sync process. Thus, it
seems 8-9M random io will be sync to the disk within 5s. Thus the
dirty data is always growing to the critical point (system
up-limitation), and then sync_entry() is always timed out. So I means,
in my case, disabling WritebackThrottle, I may always have 600 IOPS.
If enabling WritebackThrottle, IOPS always drop to 200 since fdatasync
cause back-end HDD disk overloaded.

It is true. One could probably disable wbthrottle and carefully tune the
kernel dirty_ratio and dirty_bytes. As I recall the problem though was
that it was inode writeback that was expensive, and there were not good
kernel knobs for limiting the dirty items in that cache. I would be very
interested in hearing about successes in this area.

Another promising direction is the batched fsync experiment that Dave
Chinner did a few months back. I'm not what the status is in
getting that into mainline, though, so it's not helpful anytime soon.

Post by Mark Nelson

Post by Nicheal
So I would like that we can dynamically throttle the IOPS in
FileStore. We cannot know the average sync() speed of the back-end
Store since different disk own different IO performance. However, we
can trace the average write speed in FileStore and Journal, Also, we
can know, whether start_sync() is return and finished. Thus, If this
time, Journal is writing so fast that the back-end cannot catch up the
Journal(e.g. 1000IOPS/s). We cannot Throttle the Journal speed(e.g.
800IOPS/s) in next operation interval(the interval maybe 1 to 5
seconds, in the third second, Thottle become 1000*e^-x where x is the
tick interval, ), if in this interval, Journal write reach the
limitation, the following submitting write should waiting in OSD
waiting queue.So in this way, Journal may provide a boosting IO, but
finally, back-end sync() will return and catch up with Journal become
we always slow down the Journal speed after several seconds.

Autotuning these parameters based on observed performance definitely
sounds promising!

sage
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[Nicheal]

Post by Sage Weil

Post by Mark Nelson

Post by Nicheal
Yes, Greg.
But Unix based system always have a parameter dirty_ratio to prevent
the system memory from being exhausted. If Journal speed is so fast
while backing store cannot catch up with Journal, then the backing
store write will be blocked by the hard limitation of system dirty
pages. The problem here may be that system call, sync(), cannot return
since the system always has lots of dirty pages. Consequently, 1)
FileStore::sync_entry() will be timeout and then ceph_osd_daemon
abort. 2) Even if the thread is not timed out, Since the Journal
committed point cannot be updated so that the Journal will be blocked,
waiting for the sync() return and update Journal committed point.
So the Throttle is added to solve the above problems, right?

Greg or Sam can correct me if I'm wrong, but I always thought of the
wbthrottle code as being more an attempt to smooth out spikes in write
throughput to prevent the journal from getting too far ahead of the backing
store. IE have more frequent, shorter flush periods rather than less frequent
longer ones. For Ceph that is's probably a reasonable idea since you want all
of the OSDs behaving as consistently as possible to prevent hitting the max
outstanding client IOs/Bytes on the client and starving other ready OSDs. I'm
not sure it's worked out in practice as well as it might have in theory,
though I'm not sure we've really investigated what's going on enough to be
sure.

Right. The fdatasync strategy means that the overall throughput is lower,
but the latencies are much more consistent. Without the throttling we had
huge spikes, which is even more problematic.

Post by Mark Nelson

Post by Nicheal
However, in my tested ARM ceph cluster(3nodes, 9osds, 3osds/node), it
will cause problem (SSD as journal, and HDD as data disk, fio 4k
WritebackThrottle enable: Based on blktrace, we trace the back-end
hdd io behaviour. Because of frequently calling fdatasync() in
Writeback Throttle, it cause every back-end hdd spent more time to
finish one io. This causes the total sync time longer. For example,
default sync_max_interval is 5 seconds, total dirty data in 5 seconds
is 10M. If I disable WritebackThrottle, 10M dirty data will be sync to
disk within 4 second, So cat /proc/meminfo, the dirty data of my
system is always clean(near zero). However, If I enable
WritebackThrottle, fdatasync() slows down the sync process. Thus, it
seems 8-9M random io will be sync to the disk within 5s. Thus the
dirty data is always growing to the critical point (system
up-limitation), and then sync_entry() is always timed out. So I means,
in my case, disabling WritebackThrottle, I may always have 600 IOPS.
If enabling WritebackThrottle, IOPS always drop to 200 since fdatasync
cause back-end HDD disk overloaded.

It is true. One could probably disable wbthrottle and carefully tune the
kernel dirty_ratio and dirty_bytes. As I recall the problem though was
that it was inode writeback that was expensive, and there were not good
kernel knobs for limiting the dirty items in that cache. I would be very
interested in hearing about successes in this area.

Yes, I also find that inode-writeback is definitely expensive. I try
to find out the reason and improve it but failed. Since it is quite
complex and different file-systems have different implementations to
manage its inode in VFS layer. Furthermore, the filesystem itself
maintains journal for inodes to accelerate inode writeback and keep
inode writeback atomic. I am still find more useful materials in how
the filesystem (XFS, EXT4) works in its VFS and their strategies
maintain and writeback inodes based on their source code. If any one
can suggests more relative literature?

Post by Sage Weil
Another promising direction is the batched fsync experiment that Dave
Chinner did a few months back. I'm not what the status is in
getting that into mainline, though, so it's not helpful anytime soon.

Post by Mark Nelson

Post by Nicheal
So I would like that we can dynamically throttle the IOPS in
FileStore. We cannot know the average sync() speed of the back-end
Store since different disk own different IO performance. However, we
can trace the average write speed in FileStore and Journal, Also, we
can know, whether start_sync() is return and finished. Thus, If this
time, Journal is writing so fast that the back-end cannot catch up the
Journal(e.g. 1000IOPS/s). We cannot Throttle the Journal speed(e.g.
800IOPS/s) in next operation interval(the interval maybe 1 to 5
seconds, in the third second, Thottle become 1000*e^-x where x is the
tick interval, ), if in this interval, Journal write reach the
limitation, the following submitting write should waiting in OSD
waiting queue.So in this way, Journal may provide a boosting IO, but
finally, back-end sync() will return and catch up with Journal become
we always slow down the Journal speed after several seconds.

Autotuning these parameters based on observed performance definitely
sounds promising!
sage

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[Nicheal]

Post by Mark Nelson

Post by Nicheal
Yes, Greg.
But Unix based system always have a parameter dirty_ratio to prevent
the system memory from being exhausted. If Journal speed is so fast
while backing store cannot catch up with Journal, then the backing
store write will be blocked by the hard limitation of system dirty
pages. The problem here may be that system call, sync(), cannot return
since the system always has lots of dirty pages. Consequently, 1)
FileStore::sync_entry() will be timeout and then ceph_osd_daemon
abort. 2) Even if the thread is not timed out, Since the Journal
committed point cannot be updated so that the Journal will be blocked,
waiting for the sync() return and update Journal committed point.
So the Throttle is added to solve the above problems, right?

Greg or Sam can correct me if I'm wrong, but I always thought of the
wbthrottle code as being more an attempt to smooth out spikes in write
throughput to prevent the journal from getting too far ahead of the backing
store. IE have more frequent, shorter flush periods rather than less
frequent longer ones. For Ceph that is's probably a reasonable idea since
you want all of the OSDs behaving as consistently as possible to prevent
hitting the max outstanding client IOs/Bytes on the client and starving
other ready OSDs. I'm not sure it's worked out in practice as well as it
might have in theory, though I'm not sure we've really investigated what's
going on enough to be sure.

Post by Nicheal
However, in my tested ARM ceph cluster(3nodes, 9osds, 3osds/node), it
will cause problem (SSD as journal, and HDD as data disk, fio 4k
WritebackThrottle enable: Based on blktrace, we trace the back-end
hdd io behaviour. Because of frequently calling fdatasync() in
Writeback Throttle, it cause every back-end hdd spent more time to
finish one io. This causes the total sync time longer. For example,
default sync_max_interval is 5 seconds, total dirty data in 5 seconds
is 10M. If I disable WritebackThrottle, 10M dirty data will be sync to
disk within 4 second, So cat /proc/meminfo, the dirty data of my
system is always clean(near zero). However, If I enable
WritebackThrottle, fdatasync() slows down the sync process. Thus, it
seems 8-9M random io will be sync to the disk within 5s. Thus the
dirty data is always growing to the critical point (system
up-limitation), and then sync_entry() is always timed out. So I means,
in my case, disabling WritebackThrottle, I may always have 600 IOPS.
If enabling WritebackThrottle, IOPS always drop to 200 since fdatasync
cause back-end HDD disk overloaded.

We never did a blktrace investigation, but we did see pretty bad performance
with the default wbthrottle code when it was first implemented. We ended up
raising the throttles pretty considerably in dumpling RC2. It would be
interesting to repeat this test on an Intel system.

Post by Nicheal
So I would like that we can dynamically throttle the IOPS in
FileStore. We cannot know the average sync() speed of the back-end
Store since different disk own different IO performance. However, we
can trace the average write speed in FileStore and Journal, Also, we
can know, whether start_sync() is return and finished. Thus, If this
time, Journal is writing so fast that the back-end cannot catch up the
Journal(e.g. 1000IOPS/s). We cannot Throttle the Journal speed(e.g.
800IOPS/s) in next operation interval(the interval maybe 1 to 5
seconds, in the third second, Thottle become 1000*e^-x where x is the
tick interval, ), if in this interval, Journal write reach the
limitation, the following submitting write should waiting in OSD
waiting queue.So in this way, Journal may provide a boosting IO, but
finally, back-end sync() will return and catch up with Journal become
we always slow down the Journal speed after several seconds.

I will wait for Sam's input, but it seems reasonable to me. Perhaps you
might write it up as a blueprint for CDS?

Ok, Mark. I would consider. But now, it just a basic idea. I may think
out whether we can use a AutotuningThrottle to replace the
WritebackThrottle.

Post by Mark Nelson
Mark

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