some ZFS questions

[Daniel Staal]

--As of August 26, 2014 1:51:44 AM -0500, Scott Bennett is alleged to have 
said:

> Paul Kraus <paul at kraus-haus.org> wrote:
>
>> On Aug 24, 2014, at 6:01, Scott Bennett <bennett at sdf.org> wrote:
>>
>> > Paul Kraus <paul at kraus-haus.org> wrote:
>>
>> >> I tend to agree. I do not recall off the top of my head, but I
>> >> *think* you can enable compression on a zvol, in which case you can
>> >> get that added benefit on the encrypted data, if you have the CPU
>> >> power to handle both the encryption and the compression at once
>> >> without too big a performance penalty.
>> >
>> >     That may be worth keeping in mind for the future, but my present
>> > collection of both encrypted and unencrypted file systems that are
>> > candidates to be moved into the care of ZFS are almost entirely
>> > compressed already.  Turning on compression for these would just add
>> > unnecessary CPU overhead.
>> >     Hmm...how would zvol compression work?  Would UFS2 data structures
>> > still be meaningful?  Would geli(8) or other g* utilities be able to
>> > find the last sector(s) in order to create/read their label metadata?
>>
>> OK, I just checked and my zvol *does* have a compression property, so
>> you can enable compression for zvols.
>
>      That's good to know in case I someday have other archival needs, but
> in the current situation, wouldn't buy me much, if anything, for the CPU
> costs, as noted above.

--As for the rest, it is mine.

Compression can be well worth the CPU cost, actually: In many cases using 
light/medium compression under ZFS *improves* performance, as it takes less 
time to compress/decompress the data then it does to transfer the 
uncompressed data to disk.

Experiment with your data, of course; it depends on how compressible things 
are, and your hardware.  But don't write it off just because it's a CPU 
cost - it might still be a good option.

Daniel T. Staal

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