Hi Nicheal,
1. The main purpose of journal is provide transaction semantics (prevent
partially update). Peer is not enough for this need because ceph writes all
replica at the same time, so when crush, you have no idea about which replica
has right data. For example, say if we have 2 replica, user update a 4M object
and the primary OSD crush when the first 2M was written , secondary OSD may
also failed when the first 3MB was written. So both versions in
primary/secondary are neither the new value, nor the old value, and have no way
to recover. So share the same idea as database, we need to have a journal to
support transaction and prevent this happen. For some backend support
transaction, BTRFS as an instance, we don't need a journal, we can write the
journal and data disk at the same time, the journal here is just try to help
performance, since it only do sequential write and we suspect it should be
faster than backend OSD.
2. Have you got any data to prove the O_DSYNC or fdatasync kill the
performance of journal? In our previous test, the journal SSD (use a
partition of a SSD as a journal for a particular OSD, and 4 OSD share a same
SSD) could reach its peak performance (300-400MB/s)
Xiaoxi
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Somnath Roy
Sent: Wednesday, September 17, 2014 3:30 PM
To: 姚宁; [email protected]
Subject: RE: puzzled with the design pattern of ceph journal, really ruining
performance
Hi Nicheal,
Not only recovery , IMHO the main purpose of ceph journal is to support
transaction semantics since XFS doesn't have that. I guess it can't be achieved
with pg_log/pg_info.
Thanks & Regards
Somnath
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of ??
Sent: Tuesday, September 16, 2014 11:29 PM
To: [email protected]
Subject: puzzled with the design pattern of ceph journal, really ruining
performance
Hi, guys
I analyze the architecture of the ceph souce code.
I know that, in order to keep journal atomic and consistent, the journal write
mode should be set with O_DSYNC or called fdatasync() system call after every
write operation. However, this kind of operation is really killing the
performance as well as achieving high committing latency, even if SSD is used
as journal disk. If the SSD has capacitor to keep the data safe when the system
crashes, we can set the mount option nobarrier or SSD itself will ignore the
FLUSH REQUEST. So the performance would be better.
So can it be instead by other strategies?
As far as I am concerned, I think the most important part is pg_log and
pg_info. It will guides the crashed osd recovery its objects from the peers.
Therefore, if we can keep pg_log at a consistent point, we can recovery data
without journal. So can we just use an "undo"
strategy on pg_log and neglect ceph journal? It will save lots of bandwidth,
and also based on the consistent pg_log epoch, we can always recovery data from
its peering osd, right? But this will lead to recovery more objects if the osd
crash.
Nicheal
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