[gpfsug-discuss] SSDs for data - DWPD?

[Buterbaugh, Kevin L]

Thanks for the suggestion, Simon.  Yes, we’ve looked at that, but we think that we’re going to potentially be in a situation where we’re using fairly big SSDs already.  For example, if we bought 30 6.4 TB SSDs rated at 1 DWPD and configured them as 6 4+1P RAID 5 LUNs, then we’d end up with a usable capacity of 6 * 4 * 6 = ~144 TB usable space in our “hot” pool.  That would satisfy our capacity needs and also not exceed the 1 DWPD rating of the drives.

BTW, we noticed with one particular vendor that their 3 DWPD drives were exactly 1/3rd the size of their 1 DWPD drives … which makes us wonder if that’s coincidence or not.  Anybody know for sure?

Thanks…

Kevin

> On Mar 18, 2019, at 4:13 PM, Simon Thompson <S.J.Thompson at bham.ac.uk> wrote:
> 
> Did you look at pricing larger SSDs than you need and only using partial capacity to get more DWPD out of them?
> 
> I.e. 1TB drive 3dpwd = 3TBpd
> 2TB drive (using 1/2 capacity) = 6TBpd
> 
> Simon
> ________________________________________
> From: gpfsug-discuss-bounces at spectrumscale.org [gpfsug-discuss-bounces at spectrumscale.org] on behalf of Buterbaugh, Kevin L [Kevin.Buterbaugh at Vanderbilt.Edu]
> Sent: 18 March 2019 19:09
> To: gpfsug main discussion list
> Subject: Re: [gpfsug-discuss] SSDs for data - DWPD?
> 
> Hi All,
> 
> Just wanted to follow up with the results of my survey … I received a grand total of two responses (Thanks Alex and John).  In their case, they’re using SSDs with a 10 DWPD rating.
> 
> The motivation behind my asking this question was … money!  ;-).  Seriously, 10 DWPD drives are still very expensive, while 3 DWPD drives are significantly less expensive and 1 DWPD drives are even cheaper still.  While we would NOT feel comfortable using anything less than 10 DWPD drives for metadata, we’re wondering about using less expensive drives for data.
> 
> For example, let’s just say that you’re getting ready to set up a brand new GPFS 5 formatted filesystem of 1-2 PB in size.  You’re considering having 3 pools:
> 
> 1) a metadata only system pool of 10 DWPD SSDs.  4K inodes, and a ton of small files that’ll fit in the inode.
> 2) a data only “hot” pool (i.e. the default pool for writes) of SSDs.
> 3) a data only “capacity” pool of 12 TB spinning disks.
> 
> And let’s just say that you have looked back at the historical data you’ve collected and you see that over the last 6 months or so you’ve been averaging 10-12 TB of data being written into your existing filesystem per day.  You want to do migrations between pools only on the weekends if at all possible.
> 
> 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.
> 
> Thanks…
> 
> Kevin
> 
> P.S.  I still have a couple of “outstanding issues” to respond to that I’ve posted to the list about previously:
> 
> 1) the long I/O’s we see occasionally in the output of “mmdiag —iohist” on our NSD servers.  We’re still trying to track that down … it seems to happen only with a subset of our hardware - most of the time at least - but we’re still working to track down what triggers it … i.e. at this point I can’t say whether it’s really the hardware or a user abusing the hardware.
> 
> 2) I promised to post benchmark results of 3 different metadata configs:  a) RAID 1 mirrors, b) a RAID 5 stripe, c) no RAID, but GPFS metadata replication of 3.  That benchmarking has been put on hold for reasons I can’t really discuss on this mailing list at this time … but hopefully soon.
> 
> I haven’t forgotten the above and will respond back on the list when it’s appropriate.  Thanks...
> 
> On Mar 8, 2019, at 10:24 AM, Buterbaugh, Kevin L <Kevin.Buterbaugh at Vanderbilt.Edu<mailto:Kevin.Buterbaugh at Vanderbilt.Edu>> wrote:
> 
> Hi All,
> 
> This is kind of a survey if you will, so for this one it might be best if you responded directly to me and I’ll summarize the results next week.
> 
> Question 1 - do you use SSDs for data?  If not - i.e. if you only use SSDs for metadata (as we currently do) - thanks, that’s all!  If, however, you do use SSDs for data, please see Question 2.
> 
> Question 2 - what is the DWPD (daily writes per day) of the SSDs that you use for data?
> 
> Question 3 - is that different than the DWPD of the SSDs for metadata?
> 
> Question 4 - any pertinent information in regards to your answers above (i.e. if you’ve got a filesystem that data is uploaded to only once and never modified after that then that’s useful to know!)?
> 
> Thanks…
> 
> Kevin
> 
> —
> Kevin Buterbaugh - Senior System Administrator
> Vanderbilt University - Advanced Computing Center for Research and Education
> Kevin.Buterbaugh at vanderbilt.edu<mailto:Kevin.Buterbaugh at vanderbilt.edu> - (615)875-9633
> 
> 
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