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List:       zfs-discuss
Subject:    Re: [zfs-discuss] Ideal Setup: RAID-5, Areca, etc!
From:       Robert Milkowski <milek () task ! gda ! pl>
Date:       2008-07-29 22:47:59
Message-ID: 756446744.20080729234759 () task ! gda ! pl
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Hello Bob,

Friday, July 25, 2008, 9:00:41 PM, you wrote:

BF> On Fri, 25 Jul 2008, Brandon High wrote:

>>> I am not sure if ZFS really has to wait for both sides of a mirror to
>>> finish, but even if it does, if there are enough VDEVs then ZFS can still
>>> proceed with writing.
>>
>> It would have to wait on an fsync() call, since that won't return
>> until both halves of the mirror have completed. If the cards you're
>> using have NVRAM, then they could return faster.

BF> While it is possible that the ZFS implementation does actually wait 
BF> for both drives to report that the data is written, it only has to 
BF> know that the data is committed to one drive in order to satisfy the 
BF> synchronous write expectation.  This is not the case for legacy 
BF> mirrored pairs where the disks are absolutely required to contain the 
BF> same content at the same logical locations.  ZFS does not require that
BF> disks in a mirrored pair contain identical content at all times.

AFAIK zfs does require that all writes are committed to all devices to
satisfy configured redundancy unless some of devices were marked as
failed. Otherwise, especially in sync case, you could loose data
because of a disk failure in a redundant configuration. Not to mention
other possible issues.

-- 
Best regards,
 Robert Milkowski                            mailto:milek@task.gda.pl
                                       http://milek.blogspot.com

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