Hello
Has anyone had any consideration about the usage of a single SSD for
both read and write and how that impacts the overall performance and
drive's endurance ?
I am interested to find out more in order to adjust the necessary stuff
and monitor it accordingly.
Fernando
On 14/09/2017 12:14, FERNANDO FREDIANI wrote:
Hello Coly
I didn't start this thread to provide numbers but to ask people view
on the concept and compare how flash technology works compared to how
it used to be a few years ago and I used ZFS case as an example
because people used to recommend to have separate devices until
sometime ago. My aim is to understand why this is not the
recommendation for bcache, if it already took in consideration newer
technology or if has anything else different on the way it deals with
write and read cache.
Regards,
Fernando
On 14/09/2017 12:04, Coly Li wrote:
On 2017/9/14 下午4:54, FERNANDO FREDIANI wrote:
It depends on every scenario. SSDs generally have a max throughput and
a max IOPS for read and write, but when you mix them it becomes more
difficult to measure. A typical SSDs caching device used for both
tasks will have the normal writing for doing the writeback caching,
have writes coming from the permanent storage to cache content more
popular (so to populate the cache) and will have reads to serve
content already cache to the user who requested.
Another point perhaps even more important than that is how the SSD in
question will stand for wearing. Now a days SSDs are much more
durable, specially those with higher DWPD. I read recently that newer
memory technology will do well compared to previous ones.
Hi Fernando,
It will be great if you may provide some performance numbers on ZFS (I
assume it should be ZFS since you mentioned it). I can understand the
concept, but real performance number should be more attractive for this
discussion :-)
Thanks in advance.
Coly Li
On 14/09/2017 11:45, Coly Li wrote:
On 2017/9/14 下午3:10, FERNANDO FREDIANI wrote:
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.
Hi Fernando,
Do you have some performance number to compare combined and separated
configurations on ZFS ? If the performance improvement is not from
adding one more SSD device, I don't why dedicate read/write SSDs may
help for performance. In my understanding, if any of the SSD has spared
throughput capability for read or write, mixed them together on both
SSDs may have better performance number.
Coly Li
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.
--
To unsubscribe from this list: send the line "unsubscribe linux-bcache" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
--
To unsubscribe from this list: send the line "unsubscribe linux-bcache" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
