"cache drives" [LWN.net]

"cache drives"

Posted Mar 6, 2013 17:07 UTC (Wed) by mathstuf (subscriber, #69389)
In reply to: "cache drives" by ntl
Parent article: The conclusion of the 3.9 merge window

Half of the drive is purely for cache. It's 64GB, but it's only 32GB when mounted (the rest being cache). There's no device for the "other half". I'd like to use that part of the drive in dm-cache, not the part I can use.

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"cache drives"

Posted Mar 6, 2013 18:02 UTC (Wed) by k3ninho (subscriber, #50375) [Link] (2 responses)

>it's only 32GB when mounted (the rest being cache)
The rest is intended to be used as the NAND cells wear out. The device has twice as much silicon as it needs so that it can be useful for longer, given the high read-erase patterns of scratch caches. The rest is not cache.

K3n.

"cache drives"

Posted Mar 6, 2013 18:30 UTC (Wed) by ntl (subscriber, #40518) [Link] (1 responses)

Yeah, I suspected something like this. So the drive exposes only half of its advertised capacity to the host. Assuming the touted "intelligent caching algorithms" are implemented in the drive and not the Windows-only software that OCZ provides, maybe it would compare favorably in a dm-cache configuration to conventional SSDs. Maybe not.

Anyway, the drives in the link seem to be EOL.

"cache drives"

Posted Mar 7, 2013 11:09 UTC (Thu) by Tobu (subscriber, #24111) [Link]

The caching is in their windows software, the SSD has no visibility on the HDD.

"cache drives"

Posted Mar 6, 2013 18:23 UTC (Wed) by Tobu (subscriber, #24111) [Link]

The small print on that page says it has 50% over-provisioning, so not visible outside of the SSD firmware. That must be there to compensate for the dataplex caching driver doing something not SSD-friendly, like heavy in-place rewriting (which would otherwise prevent the FTL from doing its work refreshing cells and spreading writes). bcache at least only writes whole erase blocks (1MB default bucket size), so it doesn't need that kind of overprovisioning.