some questions about transactions during write op?

[Tim Zhang]

Hi ceph,
I dump the transactions during putting a file (the size is 1M ) into
ceph, below is what I got for locat_t to store pg log and set infos.
I add some contents to see what the bufferlist has.
my understand for these four ops is:
op 0 is used to store the pglog;
op 1 is used to store epoch and pginfo;
op 2 is used to rm the trimmed pglog key(although it is empty in my example);
op 3 is used to set pglog.
Is my understanding right?
Here are some more questions:
1 why we need to store epoch and pginfo, isn't pglog enough to track
all the changes?
2 seems op 0 and op 3 is the same, isn't it?
3 I find the value of big info in leveldb is always zero, then why
we set this key?
[***@ceph_node0 ~]# ceph-kvstore-tool
/var/lib/ceph/osd/ceph-1/current/omap/ get
_USER_0000000000000001_USER_ 0.3c_biginfo
(_USER_0000000000000001_USER_, 0.3c_biginfo)
0000 : 00 00 00 00 00 00 00 00 00 00 00 00 : ............

---------------------------------------------------------------------
transaction dump for local_t:
{ "osr": "0.35",
"seq": 3669463,
"trans_num": 0,
"ops": [
{ "op_num": 0,
"op_name": "omap_setkeys",
"collection": "meta",
"oid": "5172dfbb\/pglog_0.35\/0\/\/-1",
"attr_lens": { "0000000038.00000000000000000007": 132,
"0000000038.00000000000000000007":
"buffer::list(len=132,\n\tbuffer::ptr(0~4 0x8ed5000 in raw 0x8ed5000
len 4096 nref 4),\n\tbuffer::ptr(0~124 0x9168000 in raw 0x9168000
len 496 nref 2),\n\tbuffer::ptr(4~4 0x8ed5004 in raw 0x8ed5000 len
4096 nref 4)\n)"}},
{ "op_num": 1,
"op_name": "omap_setkeys",
"collection": "meta",
"oid": "16ef7597\/infos\/head\/\/-1",
"attr_lens": { "0.35_epoch": 4,
"0.35_epoch":
"buffer::list(len=4,\n\tbuffer::ptr(0~4 0x8f24000 in raw 0x8f24000 len
4096 nref 2)\n)",
"0.35_info": 568,
"0.35_info":
"buffer::list(len=568,\n\tbuffer::ptr(0~568 0x4cae000 in raw 0x4cae000
len 4096 nref 2)\n)"}},
{ "op_num": 2,
"op_name": "omap_rmkeys",
"collection": "meta",
"oid": "5172dfbb\/pglog_0.35\/0\/\/-1",
"keys": ""},
{ "op_num": 3,
"op_name": "omap_setkeys",
"collection": "meta",
"oid": "5172dfbb\/pglog_0.35\/0\/\/-1",
"attr_lens": { "0000000038.00000000000000000007": 132,
"0000000038.00000000000000000007":
"buffer::list(len=132,\n\tbuffer::ptr(0~4 0x916ec00 in raw 0x916ec00
len 496 nref 4),\n\tbuffer::ptr(0~124 0x916f600 in raw 0x916f600
len 496 nref 2),\n\tbuffer::ptr(4~4 0x916ec04 in raw 0x916ec00 len 496
nref 4)\n)"}}]},
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