(Info) SSD Cache Endurance

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(Info) SSD Cache Endurance

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DS412+, DS1815+, DS1817+ (HA), DS1019+, DS416slim, DS920+, DS1621+ (HA)
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I found the NasCompares nvme list very informative when it came to choosing suitable drives for the new DS1621+ HA cluster I'm currently preparing for migration for our domain storage. We don't have a huge amount of data, 4.2Tb = 5x iSCSi LUN, 4.2Tb = ABB backups. About 15 months ago one or both of the SSD cache drives failed and for whatever reason it was decided to use a single Kingston SA1000 240Gb as a read only cache, cache advisor indicated 240Gb would be ideal, so went with Kingston SA1000 rated at 150TBW endurance.

All good I thought, S.M.A.R.T always showed healthy status (as with old drives) but digging deeper was surprised to find that in 435 days, it has written 254Gb a day, 107Tb in total. The upshot of this is that if I want the new NAS' SSD caches warranty to last for 5 years, then it has to have endurance of 1825 days * 0.248Tb = 453TBW which led me (via NasCompares list) choosing 2x 1Tb Seagate Firecuda which were listed as 850-3600TBW and 0.99? DWPD.

Seagate list the Firecuda 520 as having 300, 600 & 1200TBW for the 500Gb, 1Tb & 2Tb drives. For the 1Tb it equals 0.33 DWPD, which exceeds my requirements and thanks to the list I know will work - but the endurance information was way off. It also nullifies what SSD cache advisor says about sizing your cache as there are not many 240 Gb SSD with the endurance required for them to last the term (failed SSD cache in a production fileserver is a total PITA), so don't be shy people, perhaps some real world SSD cache usage will show which drives are really worth it.

In the S.M.A.R.T attibutes page of SSD, data units written x 500 = KBW / 1024 / 1024 / 1024 = TBW divide this by power on hours / 24 gives a daily cache usage. Against 8.4TB data stored 0.248TB = 2.95% of total storage used being cached per day. Totally boring topic, but more info about this topic could be helpful in long run
 

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