Hello Coly.
If the users reads a piece of data that is just writen to SSD (unlikely)
it should first and in any condition be commited to the permanent
storage and then read from there and cached in another area of the SSD.
Writaback cache is very volatile and lasts only a few seconds while the
data is not yet committed to permanent storage.
In fact multiple device suport is not implemented yet, that's why I am
asking it and comparing with other well technology as ZFS.
Regards,
Fernando
On 14/09/2017 04:58, Coly Li wrote:
On 2017/9/11 下午4:04, FERNANDO FREDIANI wrote:
Hi folks
In Bcache people normally use a single SSD for both Read and Write
cache. This seems to work pretty well, at least for the load we have
been using here.
However in other environments, specially on ZFS people tend to suggest
to use dedicated SSDs for Write (ZIL) and for Read (L2ARC). Some say
that performance will be much better in this way and mainly say they
have different wearing levels.
The issue now a days is that SSDs for Write Cache (or Writeback) don't
need to have much space available (8GB normally is more than enough),
just enough for the time until data is committed to the pool (or
slower disks) so it is hard to find a suitable SSD to dedicate to this
propose only without overprovisioning that part.
On the top of that newer SSDs have changed a lot in recent times using
different types of memory technologies which tend to be much durable.
Given that I personally see that using a single SSD for both Write and
Read cache, in any scenarios doesn't impose any significant loss to
the storage, given you use new technology SSDs and that you will
hardly saturate it most of the time. Does anyone agree or disagree
with that ?
Hi Fernando,
If there is any real performance number, it will be much easier to
response this idea. What confuses me is, if user reads a data block
which is just written to SSD, what is the benefit for the separated SSDs.
Yes I agree with you that some times a single SSD as cache device is
inefficient. Multiple cache device on bcache is a not-implemented yet
feature as I know.
Thanks.
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