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@xxxxxxxxx>:
Hi!
A. James Lewis <james@xxxxxxxxxx> 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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