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

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

[Jarry]

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

[Florian Philipp]

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Am 09.07.2012 19:26, schrieb Jarry:
> 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

I don't think trim is currently supported in mdaid. AFAIK dmraid
supports it as long as the RAID controller plays along. I'm unsure about
LVM mirrors.

But trim is not your only issue: mdraid and LVM as well as most (all?)
hardware RAIDs will mirror all blocks from one disk to the other when
you assemble the RAID-1. This means that all blocks on your second SSD
will immediately be marked dirty which incurs a significant performance
penalty.

My advice: Use the second disk to do incremental snapshots of the other
on the filesystem level. Or, if you feel lucky, use btrfs's mirroring
capabilities.

Regards,
Florian Philipp


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

[Michael Mol]

On Mon, Jul 9, 2012 at 3:12 PM, Florian Philipp <lists@binarywings.net> wrote:
> Am 09.07.2012 19:26, schrieb Jarry:
>> 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
>
> I don't think trim is currently supported in mdaid. AFAIK dmraid
> supports it as long as the RAID controller plays along. I'm unsure about
> LVM mirrors.
>
> But trim is not your only issue: mdraid and LVM as well as most (all?)
> hardware RAIDs will mirror all blocks from one disk to the other when
> you assemble the RAID-1. This means that all blocks on your second SSD
> will immediately be marked dirty which incurs a significant performance
> penalty.

Well, yes and no. Yes, there will be a performance penalty from having
all the blocks marked dirty. But this would eventually happen anyway!

On the other hand, once all blocks are marked dirty, you have
performance _stability_. If you're in a position to care about
predictive performance metrics, stability is quite useful.

-- 
:wq

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

[Volker Armin Hemmann]

Am Montag, 9. Juli 2012, 19:26:27 schrieb Jarry:
> 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

just ask intel support. Really.

-- 
#163933

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

[Jarry]

On 11-Jul-12 18:09, Volker Armin Hemmann wrote:
>>
>> Does anyone have any experience with SSD in raid (SW or HW)?
>> Is it safe to use it on server?
>
> just ask intel support. Really.

<quote>
Dear Mr...,
Thank you for contacting Intel® Technical Support.
Please be aware that we do not offer interactive presales support.
However, if you require advice regarding one of our mainstream Intel 
"boxed" products, we recommend that you contact one of our authorised 
retailers who should be in the position to help you
</quote>

Well, that is "Intel-way" of support: first you have to buy
product, and then maybe they tell you you can not use it...
:-(

Jarry

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

[Volker Armin Hemmann]

Am Freitag, 13. Juli 2012, 18:34:01 schrieb Jarry:
> On 11-Jul-12 18:09, Volker Armin Hemmann wrote:
> >> Does anyone have any experience with SSD in raid (SW or HW)?
> >> Is it safe to use it on server?
> > 
> > just ask intel support. Really.
> 
> <quote>
> Dear Mr...,
> Thank you for contacting Intel® Technical Support.
> Please be aware that we do not offer interactive presales support.
> However, if you require advice regarding one of our mainstream Intel
> "boxed" products, we recommend that you contact one of our authorised
> retailers who should be in the position to help you
> </quote>
> 
> Well, that is "Intel-way" of support: first you have to buy
> product, and then maybe they tell you you can not use it...
> 
> :-(
> 
> Jarry

hm, well... OCZ support is really nice ;)

-- 
#163933

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

[Mark Knecht]

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