Re: layering question.

A. James Lewis
2015-08-07T14:38:37Z

That's interesting, are you putting your MD on top of multiple bcache
devices... rather than bcache on top of an MD device... I wonder what
the rationale behind this is?

Also, can anyone give me a summary of how bcache compares with dm-cache?

James

On 07/08/15 13:43, Jens-U. Mozdzen wrote:
> Hi *,
>
> Zitat von Kai Krakow <hurikhan77@gmail.com>:
>> Hi!
>>
>> A. James Lewis <james@fsck.co.uk> schrieb:
>>
>>> The problem is tho... with a very large backing store, I'm not really
>>> happy with a single point of failure in the cache... is there another
>>> way to mirror the cache device?
>>
>> Well, AFAIR there are plans to add such capabilities into bcache
>> itself -
>> read: make it possible to add more than one caching device to a cache
>> set.
>> It will use some sort of hybrid mirror / striping to get the best
>> combination of speed and safety - at least that's what the idea is
>> about. I
>> just don't remember where I've read about it, neither do I know the
>> status
>> of it.
>>
>> If you want to eliminate the single point of failure, you may want to
>> try
>> mdadm with its write-mostly option instead of using bcache. It's
>> slower for
>> writes obviously but gracefully falls back if the SSD fails.
>> Obviously, you
>> can also not benefit from having a huge storage because it's classic
>> RAID-1
>> and thus the smallest member will limit your storage size.
>>
>> Bcache also has countermeasures for a failing caching device but I
>> didn't
>> really look into that yet. You should read the documentation about it in
>> Documentation/bcache.txt (Error Handling). The safest mode to use
>> here is
>> writethrough.
>
> A work of caution here: At least in my layered (kernel 3.18.8)
> situation, the upper layers from time to time run into some sort of
> time-out situation when writing to (bcached) disk. Teh writes abort
> (bad, but tolerable in my circumstances), but on top this makes MD
> mark the current disk faulty, degrading your RAID.
>
> When using "writeback", the likeliness for this to happen is
> relatively small (not more than once every few days), probably because
> the writes to SSD are fairly quick. These hit then have always been on
> the caching device (MD-RAID1 in my case).
>
> When using "writethrough", the likeliness was extremely higher (I've
> seen 2 hits within 6 hours, not later than 28 hours after switching to
> "writethrough") and the hit was on the data device (MD-RAID6 in my case).
>
> Had I only set up RAID5, my data array would have dropped dead then.
>
> After switching back to "writeback", I've had *one* further incident,
> again on the caching device, within 6 days.
>
> I would definitely not call "writethrough" "the safest mode" when
> using MD-RAID for the bcache devices, on kernel 3.18.8.
>
> Regards,
> Jens
>
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