Re: SSD usage for bcache - Read and Writeback

[FERNANDO FREDIANI <fernando.frediani@upx.com>]

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.
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