What approach did sandisk take with this for jewel?
Tyler Bishop
Chief Technical Officer
513-299-7108 x10
[email protected]
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From: "Jan Schermer" <[email protected]>
To: "Tyler Bishop" <[email protected]>
Cc: "Bill WONG" <[email protected]>, [email protected]
Sent: Thursday, January 28, 2016 4:32:54 PM
Subject: Re: [ceph-users] SSD Journal
You can't run Ceph OSD without a journal. The journal is always there.
If you don't have a journal partition then there's a "journal" file on the OSD
filesystem that does the same thing. If it's a partition then this file turns
into a symlink.
You will always be better off with a journal on a separate partition because of
the way writeback cache in linux works (someone correct me if I'm wrong).
The journal needs to flush to disk quite often, and linux is not always able to
flush only the journal data. You can't defer metadata flushing forever and also
doing fsync() makes all the dirty data flush as well. ext2/3/4 also flushes
data to the filesystem periodicaly (5s is it I think?) which will make the
latency of the journal go through the roof momentarily.
(I'll leave researching how exactly XFS does it to those who care about that
"filesystem'o'thing").
P.S. I feel very strongly that this whole concept is broken fundamentaly. We
already have a journal for the filesystem which is time proven, well behaved
and above all fast. Instead there's this reinvented wheel which supposedly does
it better in userspace while not really avoiding the filesystem journal either.
It would maybe make sense if OSD was storing the data on a block device
directly, avoiding the filesystem altogether. But it would still do the same
bloody thing and (no disrespect) ext4 does this better than Ceph ever will.
On 28 Jan 2016, at 20:01, Tyler Bishop < [email protected] >
wrote:
This is an interesting topic that i've been waiting for.
Right now we run the journal as a partition on the data disk. I've build drives
without journals and the write performance seems okay but random io performance
is poor in comparison to what it should be.
Tyler Bishop
Chief Technical Officer
513-299-7108 x10
[email protected]
If you are not the intended recipient of this transmission you are notified
that disclosing, copying, distributing or taking any action in reliance on the
contents of this information is strictly prohibited.
From: "Bill WONG" < [email protected] >
To: "ceph-users" < [email protected] >
Sent: Thursday, January 28, 2016 1:36:01 PM
Subject: [ceph-users] SSD Journal
Hi,
i have tested with SSD Journal with SATA, it works perfectly.. now, i am
testing with full SSD ceph cluster, now with full SSD ceph cluster, do i still
need to have SSD as journal disk?
[ assumed i do not have PCIe SSD Flash which is better performance than normal
SSD disk]
please give some ideas on full ssd ceph cluster ... thank you!
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