On 12/03/2013 12:26 AM, Paul B. Henson wrote:
Haven't seen much traffic on the list, maybe everybody is still recovering
from turkey comas (at least in the US ;) ).
No thoughts on this issue? Am I misunderstanding the purpose of the -w
argument, is it not intended to align with the SSD page size?
After deep digging in this area I would say that the problem is
somewhere else. All internal stuff of Linux kernel has default memory
page size of 4kB. That's way You can't use ext4 filesystem with 8kB
block size on x86 which is related to memory page size (4kB). Linus in
the mailing list said that using larger pages will be huge waste of
memory so that way the didn't do it for now. Anyway I think that with
this approach that is implemented in bcache it's not possible to use
different size of block size larger than 4kB (smaller is not the issue).
Maybe using different soft there is different approach so it is possible
to use larger block but bcache I think I use this. The another problem
is that SSD's manufacturers don't release information about some basic
info about SSD like page size or nand erase block size (I spent long
hours to get some information from forums but is not official info from
manufacturers). So that way it's hard to say if SSD disk will wait for
another 4kB of page and create 2x4kB = 8kB and write "full" page and how
it will hit performance during reads and writes. Some controllers use
compression during write so it's hard to get any info how is works.
Now I'm trying to get some info from SSD manufacturer of my SSD's disks
(I also have disks that have 8kB page size) how disk will work with
smaller size of page.
regards
Jakub Witkowski
-----Original Message-----
From: Paul Henson [mailto:paul.b.henson@xxxxxxxxx] On Behalf Of Paul B.
Henson
Sent: Friday, November 29, 2013 7:38 PM
To: linux-bcache@xxxxxxxxxxxxxxx
Subject: register_bcache() error opening /dev/md2: bad block/bucket size
I'm trying to set up a mirror of two Samsung 840 Pro 256GB SSDs as a
cache device. Per their specs, the page size is 8k, and the erase block
size is 1024k, so I created the cache device as:
make-bcache --wipe-bcache -b 1024k -w 8k -C /dev/md2
With a bucket size equal to the erase block, and a block size equal to
the page size. The documentation on block size wasn't really clear, but
I believe it's supposed to equal the ssd page size, ie, the minimum io
size for the device.
However, when trying to register this cache device, it fails with:
bcache: register_bcache() error opening /dev/md2: Bad block/bucket size
Looking at the code:
err = "Bad block/bucket size";
if (!is_power_of_2(sb->block_size) ||
sb->block_size > PAGE_SECTORS ||
!is_power_of_2(sb->bucket_size) ||
sb->bucket_size < PAGE_SECTORS)
goto err;
and given PAGE_SIZE = 4096 and PAGE_SECTORS = 8, it's complaining that my
cache device block size of 8k is greater than the page size of 4k. I guess
I could create my cache device with a block size of 4k to make it work,
but
that seems inefficient as it does not match the page size. What's this
limitation about? Can it be changed? Are SSD's with a > 4k page size just
poor choices for a cache device?
Thanks...
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