ZFS works marvelously well for data warehouse and analytic DBs. For lots of
small updates scattered across the breadth of the persistent working set, it's
not going to work well IMO.
Note that we're using ZFS to host databases as large as 10,000 TB - that's 10PB
(!!). Solaris 10 U5 on X4540. That said - it's on 96 servers running
Greenplum DB.
With SSD, the randomness won't matter much I expect, though the filesystem
won't be helping by virtue of this fragmentation effect of COW.
- Luke
----- Original Message -----
From: [EMAIL PROTECTED] <[EMAIL PROTECTED]>
To: zfs-discuss@opensolaris.org <zfs-discuss@opensolaris.org>
Sent: Sat Nov 22 16:43:53 2008
Subject: Re: [zfs-discuss] ZFS fragmentation with MySQL databases
Kees Nuyt wrote:
> My explanation would be: Whenever a block within a file
> changes, zfs has to write it at another location ("copy on
> write"), so the previous version isn't immediately lost.
>
> Zfs will try to keep the new version of the block close to
> the original one, but after several changes on the same
> database page, things get pretty messed up and logical
> sequential I/O becomes pretty much physically random indeed.
>
> The original blocks will eventually be added to the freelist
> and reused, so proximity can be restored, but it will never
> be 100% sequential again.
> The effect is larger when many snapshots are kept, because
> older block versions are not freed, or when the same block
> is changed very often and freelist updating has to be
> postponed.
>
> That is the trade-off between "always consistent" and
> "fast".
>
Well, does that mean ZFS is not best suited for database engines as
underlying
filesystem? With databases it will always be fragmented, hence slow
performance?
Because this way it would be best to use it for large file server that
don't usually change frequently.
Thanks,
Tamer
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