[zfs-discuss] L2ARC and SLOG on HW RAID with writeback cache

Gandalf Corvotempesta gandalf.corvotempesta at gmail.com
Sun Apr 29 05:39:59 EDT 2018


Il ven 27 apr 2018, 21:29 Gordan Bobic via zfs-discuss <
zfs-discuss at list.zfsonlinux.org> ha scritto:

> Using hardware RAID might seem appealing but you are going to regret it
> the first moment you get bitten by bit rot that the hardware RAID didn't
> protect you from.
>

This bitrot myth is totally nonsense today
In last 15 years i've replaced thousands of disk on hundreds or servers,
had multiple catastrophic failures (due to hw raid) but never ever seen a
single bit rot. Never.

Probably bitrot was common with disks coming from a decade ago, but current
disks (particularry with SAS interface) doesn't soffer bit rot too much (if
at all) and SAS is able to perform the check automatically thank to T10
extension

And Yes, i always check our file checksums, during the backup phase with
rsync and during the backup monthly check where we perform a total md5
check on all backup files.
If rsync doesn't transfer a file, it means that both checksum (source and
destinations) are identical and the only way to calculate a checksum is by
reading both files in both server, thus no bitrot

So, i don't think that bitrot protection is something useful today and you
always have backups for this rare corner case

ZFS is much more useful in resilvering only used space and not the whole
disks or for native compression and SSD cache

The raid part doesn't have any real advantage vs mdadm (Yes, i agree, avoid
hw raid at any cost, but for other reasons, including a huge vendor
lock-in, not for bitrot).
Honestly, mdadm is much more flexible. Using it everywhere from the
Dinosaurs era without any single issue even by mixing cheap desktop drives
with reallocated sectors (no TLER) , SSD and enterprise SAS in a single
array

>
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