Hi Brian, write endurance is one thing you need to run small IOs on on RAID5/RAID6.
However, while SSDs are much faster than HDDs when it comes to reads, they are just faster when it comes to writes. The RMW penalty on small writes to RAID5 / RAID6 will incur a higher actual data write rate at your SSD devices than you see going from your OS / file system to the storage. How much higher depends on the actual IO sizes to the RAID device related to your full stripe widths. Mind that the write caches on all levels will help here getting the the IOs larger than what the application does. Beyond a certain point, however, if you go to smaller and smaller IOs (in relation to your stripe widths) you might want to look for some other redundancy code than RAID5/RAID6 or related parity-using mechanisms even if you pay the capacity price of simple data replication (RAID1, or 3w in GNR). That depends of course, but is worth a consideration. Mit freundlichen Grüßen / Kind regards Dr. Uwe Falke IT Specialist High Performance Computing Services / Integrated Technology Services / Data Center Services ------------------------------------------------------------------------------------------------------------------------------------------- IBM Deutschland Rathausstr. 7 09111 Chemnitz Phone: +49 371 6978 2165 Mobile: +49 175 575 2877 E-Mail: [email protected] ------------------------------------------------------------------------------------------------------------------------------------------- IBM Deutschland Business & Technology Services GmbH / Geschäftsführung: Frank Hammer, Thorsten Moehring Sitz der Gesellschaft: Ehningen / Registergericht: Amtsgericht Stuttgart, HRB 17122 From: Brian Marshall <[email protected]> To: gpfsug main discussion list <[email protected]> Date: 07/18/2016 04:08 PM Subject: Re: [gpfsug-discuss] SSD LUN setup Sent by: [email protected] @Jonathan, I completely agree on the SSD failure. I wasn't suggesting that better write endurance made them impervious to failures, just that I read a few articles from ~3-5 years back saying that RAID5 or RAID6 would destroy your SSDs and have a really high probability of all SSDs failing at the same time as the # of writes were equal on all SSDs in the RAID group. I think that's no longer the case and RAID6 on SSDs is fine. I was looking for examples of what others have done: RAID6, using GPFS data replicas, or some other thing I don't know about that better takes advantage of SSD architecture. Background - I am a storage noob Also is the @Jonathan proper list etiquette? Thanks everyone to great advice I've been getting. Thank you, Brian On Sun, Jul 17, 2016 at 5:49 PM, Jonathan Buzzard <[email protected]> wrote: On 17/07/16 03:56, Brian Marshall wrote: When setting up SSDs to be used as a fast tier storage pool, are people still doing RAID6 LUNs? I think write endurance is good enough now that this is no longer a big concern (maybe a small concern). I could be wrong. I have read about other products doing RAID1 with deduplication and compression to take less than the 50% capacity hit. There are plenty of ways in which an SSD can fail that does not involve problems with write endurance. The idea of using any disks in anything other than a test/dev GPFS file system that you simply don't care about if it goes belly up, that are not RAID or similarly protected is in my view fool hardy in the extreme. It would be like saying that HDD's can only fail due to surface defects on the platers, and then getting stung when the drive motor fails or the drive electronics stop working or better yet the drive electrics go puff literately in smoke and there is scorch marks on the PCB. Or how about a drive firmware issue that causes them to play dead under certain work loads, or drive firmware issues that just cause them to die prematurely in large numbers. These are all failure modes I have personally witnessed. My sample size for SSD's is still way to small to have seen lots of wacky failure modes, but I don't for one second believe that given time I won't see them. JAB. -- Jonathan A. Buzzard Email: jonathan (at) buzzard.me.uk Fife, United Kingdom. _______________________________________________ gpfsug-discuss mailing list gpfsug-discuss at spectrumscale.org http://gpfsug.org/mailman/listinfo/gpfsug-discuss _______________________________________________ gpfsug-discuss mailing list gpfsug-discuss at spectrumscale.org http://gpfsug.org/mailman/listinfo/gpfsug-discuss _______________________________________________ gpfsug-discuss mailing list gpfsug-discuss at spectrumscale.org http://gpfsug.org/mailman/listinfo/gpfsug-discuss
