zfs discuss - Sep 2010 - Compression block sizes

When using compression, are the on-disk record sizes determined before
or after compression is applied? In other words, if record size is set
to 128k, is that the amount of data fed into the compression engine,
or is the output size trimmed to fit? I think it''s the former, but
I''m
not certain.

This could affect block alignment for SSDs, leading to worse write
performance. It could also have a negative effect on 4k sector disks
that lie about the sector size, such as the recent WD EARS drives.

-B

-- 
Brandon High : bhigh at freaks.com

On Wed, 15 Sep 2010, Brandon High wrote:
> When using compression, are the on-disk record sizes determined before
> or after compression is applied? In other words, if record size is set
> to 128k, is that the amount of data fed into the compression engine,
> or is the output size trimmed to fit? I think it''s the former, but
I''m
> not certain.
We have been told before that the blocksize is applied to the 
uncompressed data and that when compression is applied, short blocks 
may be written to disk.  This does not mean that the short blocks 
don''t start at a particular alignment.  When using raidz, the zfs 
blocks are already broken up into smaller chunks, using a smaller 
alignment than the zfs record size.  For zfs send, the data is 
uncompressed to full records prior to sending.

Bob
-- 
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/