The only way I know to actually extend the reserved space it using the method described here: https://www.thomas-krenn.com/en/wiki/SSD_Over-provisioning_using_hdparm On Wed, Apr 29, 2015 at 12:12 PM, Lionel Bouton <lionel+ceph@xxxxxxxxxxx> wrote: > Hi Dominik, > > On 04/29/15 19:06, Dominik Hannen wrote: >> I had planned to use at maximum 80GB of the available 250GB. >> 1 x 16GB OS >> 4 x 8, 12 or 16GB partitions for osd-journals. >> >> For a total SSD Usage of 19.2%, 25.6% or 32% >> and over-provisioning of 80.8%, 74.3% or 68%. >> >> I am relatively certain that those SSDs would last ages with THAT >> much over-provisioning. > > > SSD lifespan is mostly linked to the total amount of data written as > they make efforts to distribute the writes evenly on all cells. In your > case this has nothing to do with over-provisioning and everything to do > with the amount of data you will write to the OSDs backed by these journals. > > Over-provisioning can only be useful if : > - your SSD is bad at distributing writes with the space it already has > with its own over-provisioning and gets better when you add your own, > - you never touched data after the offset where your last partition ends > (or used TRIM manually to tell the SSD it's OK to use the cells they are > currently on as the SSD sees fit). > > I see people assuming it's enough to not write on the whole LBA space to > automatically get better lifespan through better data redistribution by > the SSD. It can only work if the SSD is absolutely sure that the rest of > the LBA space is unused. This can only happen if this space as never > been written to or TRIM has been used to tell it to the SSD. If you had > a filesystem full of data at the end of the SSD and removed it without > "trimming" the space, it's as if it's still there from the point of view > of your SSD. If you tested the drive with some kind of benchmark, same > problem. > If you didn't call TRIM yourself on the whole drive, you can't assume > that over-provisioning will work. It depends on how the drive as been > initialized by the manufacturer and AFAIK you don't have access to this > information (I wouldn't be surprised if someone working in marketing at > a manufacturer would think a good idea to write a default partition > scheme with NTFS filesystems without asking techs to validate the idea > and that an intern would use some tools overwriting the whole drive to > do it). > > Even with these precautions I've never seen comparative studies for > lifespans of SSD with various levels of manual over-provisioning so I'm > not even sure that this technique was ever successful (it can't really > do much harm as you don't want your journals to be too big anyway). > > I would be *very* surprised if the drive would get any performance or > life expectancy benefit when used at 19.2% instead of 32% or even 75%... > > Best regards, > > Lionel Bouton > _______________________________________________ > ceph-users mailing list > ceph-users@xxxxxxxxxxxxxx > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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