Re: Ceph and EnhanceIO cache

[Nick Fisk <nick@xxxxxxxxxx>]

I think flashcache bombs out, I must admit I have tested that yet, but as I would only be running it in writecache mode, there is no requirement I can think of for it to keep on running gracefully.

 

From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf Of Dominik Zalewski
Sent: 26 June 2015 10:54
To: Nick Fisk
Cc: ceph-users@xxxxxxxxxxxxxx
Subject: Re: [ceph-users] Ceph and EnhanceIO cache

 

Thanks for your reply. 

 

Do you know by any chance how flashcache handles SSD going offline?

 

Here is an snip from enhanceio wiki page:

 

Failure of an SSD device in read-only and write-through modes is

              handled gracefully by allowing I/O to continue to/from the

              source volume. An application may notice a drop in performance but it

              will not receive any I/O errors.

              Failure of an SSD device in write-back mode obviously results in the

              loss of dirty blocks in the cache. To guard against this data loss, two

              SSD devices can be mirrored via RAID 1.

              EnhanceIO identifies device failures based on error codes. Depending on

              whether the failure is likely to be intermittent or permanent, it takes

              the best suited action.

Looking at mailing list and github commits, both flashcache and enhanceio had not much going on since last  year.

Dominik

 

 

On Fri, Jun 26, 2015 at 10:28 AM, Nick Fisk <nick@xxxxxxxxxx> wrote:

> -----Original Message-----
> From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf Of
> Dominik Zalewski
> Sent: 26 June 2015 09:59
> To: ceph-users@xxxxxxxxxxxxxx
> Subject: Ceph and EnhanceIO cache
>
> Hi,
>
> I came across this blog post mentioning using EnhanceIO (fork of flashcache)
> as cache for OSDs.
>
> http://xo4t.mj.am/link/xo4t/jgu895v/1/DnECCniu-HWfTojpLN1IqA/aHR0cDovL3d3dy5zZWJhc3RpZW4taGFuLmZyL2Jsb2cvMjAxNC8xMC8wNi9jZXBoLWFuZC1lbmhhbmNlaW8v
>
> http://xo4t.mj.am/link/xo4t/jgu895v/2/FTxs29ShRIqNOekTAo2wKw/aHR0cHM6Ly9naXRodWIuY29tL3N0ZWMtaW5jL0VuaGFuY2VJTw
>
> I'm planning to test it with 5x 1TB HGST Travelstar 7k1000 2.5inch OSDs and
> using 256GB Transcend SSD as enhanceio cache.
>
> I'm wondering if anyone is using EnhanceIO or others like bcache, dm-cache
> with Ceph in production and what is your experience/results.

Not using in production, but have been testing all of the above both caching the OSD and RBD's.

If your RBD's are being used in scenarios where small sync writes are important (iscsi,database's) then caching the RBD's is almost essential. My findings:-

FlashCache - Probably the best of the bunch. Has sequential override and hopefully facebook will continue to maintain it
EnhanceIO - Nice that you can hot add the cache, but is likely to no longer be maintained, so risky for production
DMCache - Well maintained, but biggest problem is that it only caches writes for blocks that are already in cache
Bcache - Didn't really spend much time looking at this. Looks as if development activity is dying down and potential stability issues
DM-WriteBoost - Great performance, really suits RBD requirements. Unfortunately the ram buffering part seems to not play safe with iSCSI use.

With something like flashcache on a RBD I can max out the SSD with small sequential write IO's and it then passes them to the RBD in nice large IO's. Bursty random writes also benefit.

Caching the OSD's, or more specifically a small section where the levelDB lives can greatly improve small block write performance. Flashcache is best for this as you can limit the sequential cutoff to the levelDB transaction size. DMcache is potentially an option as well. You can probably halve OSD latency by doing this.

>
> Thanks
>
> Dominik

 

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