[zfs-discuss] recommendations for a 50 TB zpool?

Gordan Bobic gordan.bobic at gmail.com
Sun Nov 4 13:24:34 EST 2018


On Sun, 4 Nov 2018, 18:03 Richard Elling <richard.elling at richardelling.com
wrote:

>
>
> On Nov 4, 2018, at 4:54 PM, Gordan Bobic <gordan.bobic at gmail.com> wrote:
>
>
>
> On Sun, 4 Nov 2018, 16:48 Richard Elling <richard.elling at richardelling.com
> wrote:
>
>>
>> Note that random write I/O performance will be 120 IOPS per vdev (480 in
>> first case, 240 in the other). It will be better for reads at approximately
>> 720 and 300 respectively. Do some testing and maths to see whether that
>> will suffice for your workload.
>>
>>
>> You’ve got this backwards: raidz does well on random writes.
>>
>
> It always writes a full stripe - so it only gets 1 disk's worth of IOPS
> per vdev. Regular parity raid typically get half of that because it
> requires RMW for partial writes. RAIDZn does away with that by always
> writing a full stripe.
>
>
> for raidz, “full stripe” doesn’t mean “all disks.” But more importantly,
> writes are most commonly allocated sequentially and aggregated. You’ll only
> really see small random writes on very full and very fragmented pools.
>

That very much depends on where fsync() happens. What you are saying only
holds true with sync=disabled.
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