Re: SSD usage for bcache - Read and Writeback

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

Hello Michael.

Yeah, it does make sense your point of view. Also agree RAID-1 is 
unnecessary.

Fernando


On 26/09/2017 16:51, Michael Lyle wrote:
> Fernando--
>
> I don't think it really matters.  Before, when capacities of SSD were
> really small and endurance was a big concern it made sense to have a
> separate write cache made out of SLC flash-- now, being able to
> wear-level over an entire large MLC device is where the longevity
> comes from.  So I understand why ZFS made the tradeoffs they did (also
> the read path / write path functionality were added at different times
> by different people)-- but I don't think you'd make the same choices
> in implementation today.
>
> As Coly points out, there's a small benefit to having different
> redundancy policies-- you don't need RAID-1 for read cache because
> it's not too big of a deal if you lose it.  But handling this
> properly-- having multiple cache devices and ensuring that not-dirty
> data has only one copy and dirty data has multiple copies-- is fairly
> complicated for various reasons.  And having separate devices IMO is
> not a good idea today-- it's both complicated to deploy and means that
> you concentrate most of the writes to one disk (e.g. you don't
> wear-level over all of the disk capacity).
>
> Mike
>
> On Tue, Sep 26, 2017 at 12:28 PM, FERNANDO FREDIANI
> <fernando.frediani@upx.com> wrote:
>> 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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