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.
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