On Oct 11, 2012, at 2:58 PM, Phillip Wagstrom <phillip.wagst...@gmail.com>
> On Oct 11, 2012, at 4:47 PM, andy thomas wrote:
>> According to a Sun document called something like 'ZFS best practice' I read
>> some time ago, best practice was to use the entire disk for ZFS and not to
>> partition or slice it in any way. Does this advice hold good for FreeBSD as
> My understanding of the best practice was that with Solaris prior to
> ZFS, it disabled the volatile disk cache.
This is not quite correct. If you use the whole disk ZFS will attempt to enable
write cache. To understand why, remember that UFS (and ext, by default) can die
horrible death (+fsck) if there is a power outage and cached data is not
flushed to disk.
So by default, Sun shipped some disks with write cache disabled by default. For
disks, they are most often shipped with write cache enabled and the most
systems (NTFS) properly issue cache flush requests as needed (for the same
issues cache flush requests).
> With ZFS, the disk cache is used, but after every transaction a cache-flush
> command is issued to ensure that the data made it the platters.
Write cache is flushed after uberblock updates and for ZIL writes. This is
uberblock updates, so the uberblock doesn't point to a garbaged MOS. It is
for ZIL writes, because they must be guaranteed written to media before ack.
> If you slice the disk, enabling the disk cache for the whole disk is
> dangerous because other file systems (meaning UFS) wouldn't do the
> cache-flush and there was a risk for data-loss should the cache fail due to,
> say a power outage.
> Can't speak to how BSD deals with the disk cache.
>> I looked at a server earlier this week that was running FreeBSD 8.0 and had
>> 2 x 1 Tb SAS disks in a ZFS 13 mirror with a third identical disk as a
>> spare. Large file I/O throughput was OK but the mail jail it hosted had
>> periods when it was very slow with accessing lots of small files. All three
>> disks (the two in the ZFS mirror plus the spare) had been partitioned with
>> gpart so that partition 1 was a 6 GB swap and partition 2 filled the rest of
>> the disk and had a 'freebsd-zfs' partition on it. It was these second
>> partitions that were part of the mirror.
>> This doesn't sound like a very good idea to me as surelt disk seeks for swap
>> and for ZFS file I/O are bound to clash. aren't they?
> It surely would make a slow, memory starved swapping system even
> slower. :)
>> Another point about the Sun ZFS paper - it mentioned optimum performance
>> would be obtained with RAIDz pools if the number of disks was between 3 and
>> 9. So I've always limited my pools to a maximum of 9 active disks plus
>> spares but the other day someone here was talking of seeing hundreds of
>> disks in a single pool! So what is the current advice for ZFS in Solaris and
> That number was drives per vdev, not per pool.
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