On 8 September 2017 at 11:00, Valeri Galtsev <galtsev@xxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, September 8, 2017 9:48 am, hw wrote:
>> m.roth@xxxxxxxxx wrote:
>>> hw wrote:
>>>> Mark Haney wrote:
>>> <snip>
>>>>> BTRFS isn't going to impact I/O any more significantly than, say, XFS.
>>>>
>>>> But mdadm does, the impact is severe. I know there are ppl saying
>>>> otherwise, but I´ve seen the impact myself, and I definitely don´t
>>>> want
>>>> it on that particular server because it would likely interfere with
>>>> other services.
>>> <snip>
>>> I haven't really been following this thread, but if your requirements
>>> are
>>> that heavy, you're past the point that you need to spring some money and
>>> buy hardware RAID cards, like LSI, er, Avago, I mean, who's bought them
>>> more recently?
>>
>> Heavy requirements are not required for the impact of md-RAID to be
>> noticeable.
>>
>> Hardware RAID is already in place, but the SSDs are "extra" and, as I
>> said,
>> not suited to be used with hardware RAID.
>
> Could someone, please, elaborate on the statement that "SSDs are not
> suitable for hardware RAID".
>
It will depend on the type of SSD and the type of hardware RAID. There
are at least 4 different classes of SSD drives with different levels
of cache, write/read performance, number of lifetime writes, etc.
There are also multiple types of hardware RAID. A lot of hardware RAID
will try to even out disk usage in different ways. This means 'moving'
the heavily used data from slow parts to fast parts etc etc. On an SSD
all these extra writes aren't needed and so if the hardware RAID
doesn't know about SSD technology it will wear out the SSD quickly.
Other hardware raid parts that can cause faster failures on SSD's are
where it does test writes all the time to see if disks are bad etc.
Again if you have gone with commodity SSD's this will wear out the
drive faster than expected and boom bad disks.
That said, some hardware RAID's are supposedly made to work with SSD
drive technology. They don't do those extra writes, they also assume
that the disks underneath will read/write in near constant time so
queueing of data is done differently. However that stuff costs extra
money and not usually shipped in standard OEM hardware.
> Thanks.
> Valeri
>
>>
>> It remains to be tested how the hardware RAID performs, which may be even
>> better than the SSDs.
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>
>
> ++++++++++++++++++++++++++++++++++++++++
> Valeri Galtsev
> Sr System Administrator
> Department of Astronomy and Astrophysics
> Kavli Institute for Cosmological Physics
> University of Chicago
> Phone: 773-702-4247
> ++++++++++++++++++++++++++++++++++++++++
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