On Wed, Oct 05, 2011 at 08:19:20AM +0400, Jim Klimov wrote:
> Hello, Daniel,
> Apparently your data is represented by rather small files (thus
> many small data blocks)

It's a zvol, default 8k block size, so yes.

> , so proportion of metadata is relatively
> high, and your<4k blocks are now using at least 4k disk space.
> For data with small blocks (a 4k volume on an ashift=12 pool)
> I saw metadata use up most of my drive - becoming equal to
> data size.

That's pretty much my assumption, yes.

> Just for the sake of completeness, I brought up a similar problem
> and a non-intrusive (compatibility-wise) solution in this bug:
> https://www.illumos.org/issues/1044
> Main idea was to let ZFS users specify a minumum data block
> size and alignment, while formal ashift=9 remains in place and
> metadata blocks remain 512b. This fix would be a code-only
> change without on-disk-format changes. 

Well, other than needing to re-make a pool already created with
ashift=12..   By the time I do that, I might as well remake it onto
5k3000's instead.. :)  

> There could be some
> further cleverness when working with (updating) metadata,
> so that hardware 4k blocks would need to be rewritten as
> rarely and as fully as possible - to reduce wear and increase
> efficiency - but the main idea is hopefully simple.

If only the implementation were too..


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