[zfs-discuss] Pure SSD Pool

Brandon High
Tue, 12 Jul 2011 19:38:55 -0000

On Tue, Jul 12, 2011 at 12:14 PM, Eric Sproul <esproul at omniti.com>
wrote:
> I see, thanks for that explanation. ?So finding drives that keep more
> space in reserve is key to getting consistent performance under ZFS.

More spare area might give you more performance, but the big
difference is the lifetime of the device. A device with more spare
area can handle more writes.

In the capacity range (eg: 50-64 GB, 64 GiB flash), then the drive
with more spare will last longer but may not offer a performance
benefit. Higher capacity drives will offer better performance because
they have more flash channels to write to, and they should last longer
because while the spare area is the same percentage of total capacity,
it''s numerically larger.

A "consumer" 240GB drive (256GiB flash) will have 27GiB spare area. An
"enterprise" 50GB (64GiB flash) drive will have 16 GiB spare area, or
about 25% of the total capacity. Even though the consumer drive only
sets aside ~ 10% for spare, it''s so much larger that it will last
longer at any given rate of writing. If you were to completely fill
and re-fill each drive, the consumer drive will fail earlier, but
you''d have to write nearly 5x as much data to fill it even once.

-B

--
Brandon High : bhigh at freaks.com