Date: Tue, 26 Aug 2014 13:54:44 -0400 From: Paul Kraus <paul@kraus-haus.org> To: FreeBSD Questions !!!! <freebsd-questions@freebsd.org> Subject: Re: some ZFS questions Message-ID: <7432CA21-8F02-4A81-9BD7-CE435EEC992D@kraus-haus.org> In-Reply-To: <C0A451924D1D1C5649C27787@[192.168.1.50]> References: <201408070816.s778G9ug015988@sdf.org> <27DAA821-0303-4D51-ADA7-7780DB8FE85D@kraus-haus.org> <201408210837.s7L8bm01019230@sdf.org> <9207FB2C-5EDE-49A7-9B0E-7C9839250A7E@kraus-haus.org> <201408241001.s7OA19dZ004925@sdf.org> <5C83C4FD-571B-4557-8AD7-5578276D2ED5@kraus-haus.org> <201408260651.s7Q6pijc023521@sdf.org> <C0A451924D1D1C5649C27787@[192.168.1.50]>
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On Aug 26, 2014, at 13:33, Daniel Staal <DStaal@usa.net> wrote: > 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. The issue comes down to three things: 1) How compressible is *your* data? Scott said that his data is already = compressed, so that probably means that ZFS compression will not achieve = much. 2) How much excess CPU capacity do you have and how fast is it? If your = CPUs are idle 80% of the time and they are fast, then they may decrease = the amount of data to be written by enough to make it worthwhile. 3) How fast is your storage? If your storage system is faster than your = *excess* CPU capacity, then compression may not get you any performance = boost. You are balancing the amount by which the compression will reduce the = amount of data to be written (and later read) by the additional CPU = overhead involved in the compression. Also remember that the speed of = your storage will probably be different for writes vs. reads. If your = data access is 95% read, then optimizing writes through compression may = not be the best answer. > 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. Excellent advice. If I can, I test the compressibility of my data sets, = often with unexpected results. For example, I discovered that raw audio = files (WAV or AIFF) do NOT compress very much using the default ZFS = compression, so I do not enable compression on datasets where those = files reside. On the other hand, I get between 2:1 and 3:1 compression = on virtual machine disk images (even static ones), so I enable = compression on those datasets. One more argument for separate datasets based on the data type. -- Paul Kraus paul@kraus-haus.org
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