[linux-lvm] Network-attached block storage and local SSDs for dm-cache

[linux-lvm] Network-attached block storage and local SSDs for dm-cache

[Konstantin Ryabitsev]

Hi, all:

I know it's possible to set up dm-cache to combine network-attached 
block devices and local SSDs, but I'm having a hard time finding any 
first-hand evidence of this being done anywhere -- so I'm wondering if 
it's because there are reasons why this is a Bad Idea, or merely because 
there aren't many reasons for folks to do that.

The reason why I'm trying to do it, in particular, is for 
mirrors.kernel.org systems where we already rely on dm-cache to combine 
large slow spinning disks with SSDs to a great advantage. Most hits on 
those systems are to the same set of files (latest distro package 
updates), so dm-cache hit-to-miss ratio is very advantageous. However, 
we need to build newest iterations of those systems, and being able to 
use network-attached storage at providers like Packet with local SSD 
drives would remove the need for us to purchase and host huge drive 
arrays.

Thanks for any insights you may offer.

Best,
Konstantin

Re: [linux-lvm] Network-attached block storage and local SSDs for dm-cache

[Mike Snitzer]

On Fri, Apr 19 2019 at  3:30pm -0400,
Konstantin Ryabitsev <konstantin@linuxfoundation.org> wrote:

> Hi, all:
> 
> I know it's possible to set up dm-cache to combine network-attached
> block devices and local SSDs, but I'm having a hard time finding any
> first-hand evidence of this being done anywhere -- so I'm wondering
> if it's because there are reasons why this is a Bad Idea, or merely
> because there aren't many reasons for folks to do that.
> 
> The reason why I'm trying to do it, in particular, is for
> mirrors.kernel.org systems where we already rely on dm-cache to
> combine large slow spinning disks with SSDs to a great advantage.
> Most hits on those systems are to the same set of files (latest
> distro package updates), so dm-cache hit-to-miss ratio is very
> advantageous. However, we need to build newest iterations of those
> systems, and being able to use network-attached storage at providers
> like Packet with local SSD drives would remove the need for us to
> purchase and host huge drive arrays.
> 
> Thanks for any insights you may offer.

Only thing that could present itself as a new challenge is the
reliability of the network-attached block devices (e.g. do network
outages compromise dm-cache's ability to function).

I've not done any focused testing for, or thinking about, the impact
unreliable block devices might have on dm-cache (or dm-thinp, etc).
Usually we advise people to ensure the devices that they layer upon are
adequately robust/reliable.  Short of that you'll need to create your
own luck by engineering a solution that provides network storage
recovery.

If the "origin" device is network-attached and proves unreliable you
can expect to see the dm-cache experience errors.  dm-cache is not
raid.  So if concerned about network outages you might want to (ab)use
dm-multipath's "queue_if_no_path" mode to queue IO for retry once the
network-based device is available again (dm-multipath isn't raid
either, but for your purposes you need some way to isolate potential for
network based faults).  Or do you think you might be able to RAID1 or
RAID5 N of these network attached drives together?

Mike

Re: [linux-lvm] Network-attached block storage and local SSDs for dm-cache

[Zdenek Kabelac]

Dne 19. 04. 19 v 21:30 Konstantin Ryabitsev napsal(a):
> Hi, all:
> 
> I know it's possible to set up dm-cache to combine network-attached block 
> devices and local SSDs, but I'm having a hard time finding any first-hand 
> evidence of this being done anywhere -- so I'm wondering if it's because there 
> are reasons why this is a Bad Idea, or merely because there aren't many 
> reasons for folks to do that.
> 
> The reason why I'm trying to do it, in particular, is for mirrors.kernel.org 
> systems where we already rely on dm-cache to combine large slow spinning disks 
> with SSDs to a great advantage. Most hits on those systems are to the same set 
> of files (latest distro package updates), so dm-cache hit-to-miss ratio is 
> very advantageous. However, we need to build newest iterations of those 
> systems, and being able to use network-attached storage at providers like 
> Packet with local SSD drives would remove the need for us to purchase and host 
> huge drive arrays.
> 
> Thanks for any insights you may offer.


Hi

 From lvm2 POV - if you put both devices into single VG - you should be able 
to easily configure the setup in a way, that your 'main/origin' LV sitting on 
network storage and  cache is setup to be located on SSD.

lvcreate -LMAXSIZE --name MYLV vg  /dev/networkshdd
lvcreate --cache -Lcachesize  vg/MYLV  /dev/ssd

But of course as Mike points out - cache currently expects the origin device 
is reliable one.

Regards

Zdenek

Re: [linux-lvm] Network-attached block storage and local SSDs for dm-cache

[Konstantin Ryabitsev]

On Mon, Apr 22, 2019 at 02:25:44PM -0400, Mike Snitzer wrote:
>> I know it's possible to set up dm-cache to combine network-attached
>> block devices and local SSDs, but I'm having a hard time finding any
>> first-hand evidence of this being done anywhere -- so I'm wondering
>> if it's because there are reasons why this is a Bad Idea, or merely
>> because there aren't many reasons for folks to do that.
>>
>> The reason why I'm trying to do it, in particular, is for
>> mirrors.kernel.org systems where we already rely on dm-cache to
>> combine large slow spinning disks with SSDs to a great advantage.
>> Most hits on those systems are to the same set of files (latest
>> distro package updates), so dm-cache hit-to-miss ratio is very
>> advantageous. However, we need to build newest iterations of those
>> systems, and being able to use network-attached storage at providers
>> like Packet with local SSD drives would remove the need for us to
>> purchase and host huge drive arrays.
>>
>> Thanks for any insights you may offer.
>
>Only thing that could present itself as a new challenge is the
>reliability of the network-attached block devices (e.g. do network
>outages compromise dm-cache's ability to function).

I expect them to be *reasonably* reliable, but of course the chances of 
network-attached block storage becoming unavailable are higher than for 
directly-attached storage.

>I've not done any focused testing for, or thinking about, the impact
>unreliable block devices might have on dm-cache (or dm-thinp, etc).
>Usually we advise people to ensure the devices that they layer upon are
>adequately robust/reliable.  Short of that you'll need to create your
>own luck by engineering a solution that provides network storage
>recovery.

I expect that in writethrough mode the worst kind of recovery we'd have 
to deal with is rebuilding the dm-cache setup, as even if the underlying 
slow storage becomes unavailable, that shouldn't result in FS corruption 
on it. Even though mirrors.kernel.org data is just that, mirrors, we 
certainly would like to avoid situations where we have to re-sync 40TB 
all over, as that usually means a week-long outage.

>If the "origin" device is network-attached and proves unreliable you
>can expect to see the dm-cache experience errors.  dm-cache is not
>raid.  So if concerned about network outages you might want to (ab)use
>dm-multipath's "queue_if_no_path" mode to queue IO for retry once the
>network-based device is available again (dm-multipath isn't raid
>either, but for your purposes you need some way to isolate potential for
>network based faults).  Or do you think you might be able to RAID1 or
>RAID5 N of these network attached drives together?

I don't think that makes sense, as these volumes would likely be coming 
from the same NAS array, so we'd be increasing complexity without 
necessarily hedging any risks.

Thanks for your help -- I think we're going to try this out as 
experimental setup and then see what kind of issue we run into.

Best,
-K