[gpfsug-discuss] SSDs for data - DWPD?

[Jonathan Buzzard]

On Mon, 2019-03-18 at 19:09 +0000, Buterbaugh, Kevin L wrote:

[SNIP]

> 
> 12 * 7 = 84 TB.  So if you had somewhere between 125 - 150 TB of SSDs
> ... 1 DWPD SSDs … then in theory you should easily be able to handle
> your anticipated workload without coming close to exceeding the 1
> DWPD rating of the SSDs.
> 
> However, as the saying goes, while in theory there’s no difference
> between theory and practice, in practice there is ... so am I
> overlooking anything here from a GPFS perspective???
> 
> If anybody still wants to respond on the DWPD rating of the SSDs they
> use for data, I’m still listening.

I would be weary of write amplification in RAID coming to bite you in
the ass. Just because you write 1TB of data to the file system does not
mean the drives write 1TB of data, it could be 2TB of data.

I would if you can look at the data written to the drives using
smartctl if you are on a DSS or ESS or something similar if they are
behind a conventional storage array.

So for example on my DSS-G picking a random drive used for data which
is an 8TB NL-SAS for the record shows the following in the output of
smartctl -a

Error counter log:
           Errors Corrected by           Total   Correction     Gigabytes    Total
               ECC          rereads/    errors   algorithm      processed    uncorrected
           fast | delayed   rewrites  corrected  invocations   [10^9 bytes]  errors
read:   682040270        0         0  682040270          0     116208.442           0
write:         0        0         0         0          0      34680.694           0


Looking at the gigabytes processed shows that 33TB has been written to
the drive. These are lifetime figures for the drive, so there is no
under reporting/estimation going on.

If you can get these figures back you can calculate what drive writes
you need because they encapsulate the RAID write amplification.

JAB.

-- 
Jonathan A. Buzzard                         Tel: +44141-5483420
HPC System Administrator, ARCHIE-WeSt.
University of Strathclyde, John Anderson Building, Glasgow. G4 0NG