Well the block-size == the physical block size your bcache device will have. (As in 4k sector disks VS 512bytes disks) Please note that the physical-sector size (as advertised by your bcache device the OS and FS) can have an impact on using it: - FS might refuse to use smaller blocks then the sector size. - When using lvm, the sector-size will the biggest of all the pv's used. (AND can change while you are doing an lvextend or pvmove) - Possible other I don't know about. I don't know if the block-size matters for performance, but usually the above points, trump performance. Bucket-size should match erase size. How important is it ? Depend on how much your SSD cares about matched erase sizes. Killian De Volder On 13-08-15 12:23, Tim Small wrote: > Hi, > > I couldn't find much in the way of docs on the block and bucket sizes... > > I created a bcache device (md 3 disk RAID5 backing, Intel S3500 cache), > and initially used the default bucket and block sizes. > > It looks like flash erase block sizes are now almost universally larger > than the bcache default bucket size, so if this is important (and the > man page says it is), then maybe this needs to be increased? > > After a load of googling, I think that for this SSD (which uses > Intel/Micron 20nm MLC), the page size is probably 8 kB, and the erase > block size is probably 256 x 8 kB = 2 MB > > http://www.anandtech.com/show/7147/micron-announces-16nm-128gb-mlc-nand-ssds-in-2014 > > - if on the other hand it uses 128 Gbit parts, then this will be 16 kB > page size, and 8 MB erase block. > > > So, after playing around a bit, I take it that: > > The block size for the backing and cache devices must be the same (are > there any implications e.g. file system compatibility - with block sizes > larger than 4 kB?). > > The default bucket size is smaller than the erase block size on this SSD > (and probably most modern SSDs), and I was wondering if the default > should be increased? > > I'm assuming most users are going to be getting these parameters "wrong" > - but I'm not sure how much impact this will have on performance and SSD > endurance? Does this need some sort of wiki -type table with a lookup > between SSD model number and page/block size (which make-bcache could use)? > > It'll be a bit of a pain to move everything off my 512 byte block size > backing store, and then recreate it, so should I bother? > > Tim. > -- > To unsubscribe from this list: send the line "unsubscribe linux-bcache" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
