Re: [gentoo-user] OT: HW-raid1 & SSD?

[Mark Knecht <markknecht@gmail.com>]

On Mon, Jul 9, 2012 at 10:26 AM, Jarry <mr.jarry@gmail.com> wrote:
> Hi,
>
> I'm building small server for VPS-hosting purpose (not decided
> yet, probably KVM or ESXi). Althought it is "non production",
> reliability is still issue for me. I'm considering using two
> small SSD (ie. Intel 313, 24GB, SLC) in hw-raid1 (Adaptec 3805)
> just for hypervisor, and 2xHDD/raid1 for VPS.
>
> I started looking for info on internet but results are not
> conclusive. Opinions vary from "no problem, use SSD&raid1"
> to "using SSD in raid is not recommended no matter if it is
> SW or HW".  Mostly missing trim-command is mentioned as reason
> for not using SSD in any raid.
>
> Does anyone have any experience with SSD in raid (SW or HW)?
> Is it safe to use it on server?
>
> Jarry

On the surface it doesn't make a lot of sense to me. If you have two
drives mapped against each other in a RAID-1, and then you TRIM part
of 1 drive but don't TRIM the same part of the 2nd drive, it seems to
me that you're discarding part of your redundancy, aren't you? What
would mdadm itself (or anything else) know about remapping that stuff?
Granted, mdadm could be informed and probably reconstruct the RAID-1
in a new location, but as you TRIM out different parts of each SSD
it's going to get messy I think.

If you look up TRIM in Wikipedia there's a little bit of info
suggesting some support for some file systems.

http://en.wikipedia.org/wiki/TRIM

I can say from reading the mdadm list for the last year that it's a
conversation that doesn't come up all that often and mostly goes away
pretty fast most of the time there. Check their archives.

In my case I'm running a small server here that's currently running 3
VMs which is typical for my work day. I'm considering adding a single
SSD to the system to host the VMs but using rsync to back them up to
HDD RAID each evening.

HTH,
Mark