[CentOS] stripe size for SSDs? ( cyrus spool on btrfs?)

Wed Sep 13 16:00:13 UTC 2017
hw <hw at gc-24.de>

Stephen John Smoogen wrote:
> On 13 September 2017 at 09:25, hw <hw at gc-24.de> wrote:
>> John R Pierce wrote:
>>> On 9/9/2017 9:47 AM, hw wrote:
>>>> Isn´t it easier for SSDs to write small chunks of data at a time?
>>>> The small chunk might fit into some free space more easily than
>>>> a large one which needs to be spread out all over the place.
>>> the SSD collects data blocks being written and when a full flash block
>>> worth of data is collected, often 256K to several MB, it writes them all at
>>> once to a single contiguous block on the flash array, no matter what the
>>> 'address' of the blocks being written is.  think of it as a 'scatter-gather'
>>> operation.
>>> different drive brands and models use different strategies for this, and
>>> all this is completely opaque to the host OS so you really can't outguess or
>>> manage this process at the OS or disk controller level.
>> What if the collector is full?
>> I understand that using small chunk sizes can reduce performance because
>> many chunks need to be dealt with.  Using large chunks would involve
>> reading and writing larger amounts of data every time, and that also
>> could reduce performance.
>> With a chunk size of 1MB, disk access might amount to huge amounts of
>> data being read and written unnecessarily.  So what might be a good chunk
>> size for SSDs?
> It will depend on the type of SSD. Ones with large cache and various
> smarts (SAS Enterprise type) can take many different sizes. For SATA
> ones it depends on what the cache and write of the SSD is and very few
> of them seem to be the same. The SSD also has all kinds of logic which
> moves data around constantly on disk to wipe level so it makes it
> opaque. The people who have tested this usually have to burn through
> an SSD set to get an idea about a particular 'run' of a model but it
> doesn't go over every version of the model of SATA SSD.

Hm, so much to SSDs ...  I can only hope they will be replaced with
something better.

I have decided against putting anything onto these SSDs other than temporary
data, but even for that, I would need to make an md-RAID, which I don´t want.
It may work or not, and "may work" is not enough.

If the performance on the hardware RAID isn´t as good, it can not get worse
than it is now, and it may be even better than with the SSDs.

I have two at home with the system installed on btrfs.  I´m going to change that
to md-RAID1 and xfs.  Is there anything special involved in copying the system
to another disk?  Will 'cp -ax' do, or should I use rsync to copy xattrs etc.?
Using the commonly used stripe size of 128kb is something I´d expect the SSDs
being able to handle.