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Linux 3.3 Kernel: Btrfs vs. EXT4

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  • #16
    Originally posted by liam View Post
    It's not visible to the file system, but if you have a SF controller you shouldn't run filesystem level compression b/c it's superfluous. At best you may save a bit of space, but it will, in all cases, slow down your ops.
    What is it I'm missing?
    Since SSDs are tiny, I don't mind having compression enabled sometimes saving ~50% of space on a filesystem, with non-noticeable performance impact.

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    • #17
      Originally posted by fackamato View Post
      Since SSDs are tiny, I don't mind having compression enabled sometimes saving ~50% of space on a filesystem, with non-noticeable performance impact.
      Except that on a Sandforce SSD, when it does save noticeable amounts of space, it will have a noticeable performance impact.

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      • #18
        Originally posted by AnonymousCoward View Post
        Except that on a Sandforce SSD, when it does save noticeable amounts of space, it will have a noticeable performance impact.
        Why won't it save space? I'm not saying you're wrong, I just don't understand it. Example:

        100GB SF SSD, 1 single btrfs partition. So let's say ~95GB or so available to the user. If the user writes 95GB of iso files with no compression, the performance while writing will be good and he'll use the entire filesystem.
        If he choses to mount the filesystem with some compression option (lzo or something as fast) then the write performance will suffer a bit (at ~ 500MB/s, who cares? It's a tradeoff I guess.), but the user will still have some free space on the file system, depending on how good lzo is at compressing these ISO images. Therefore, the user can store more data than the SSD is physically capable of.

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        • #19
          http://marc.info/?l=linux-ext4&m=133052231227201&w=2

          oh look, btrfs a lot faster than ext4 in a real world example.

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          • #20
            Originally posted by fackamato View Post
            Since SSDs are tiny, I don't mind having compression enabled sometimes saving ~50% of space on a filesystem, with non-noticeable performance impact.
            with an SF controller who does compression you do not gain any space - but you are wearing the SSD down if you compress the data. Because the controller will not be able to compress it further, defeating one of the wear leveling mechanism.

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            • #21
              Originally posted by energyman View Post
              with an SF controller who does compression you do not gain any space - but you are wearing the SSD down if you compress the data. Because the controller will not be able to compress it further, defeating one of the wear leveling mechanism.
              So if you write 50GB to the SF SSD without compression, you'll see less than 50GB used on the filesystem?

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              • #22
                no, you see 50gb used. But internally it is only say 40 gb, saving the flash chips from some dangerous write cycles.

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                • #23
                  Originally posted by energyman View Post
                  no, you see 50gb used. But internally it is only say 40 gb, saving the flash chips from some dangerous write cycles.
                  Exactly. Personally I'd use compression anyway to save space, unless it hurt write performance by >50%. The lifetime of the SSDs is fine anways regardless, so this is the route I'd go

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                  • #24
                    emm, no - the write cycles are not 'ok'. The smaller the structure the less write cycles you get. Modern flash chips write cycles are pathetic.

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                    • #25
                      Originally posted by energyman View Post
                      http://marc.info/?l=linux-ext4&m=133052231227201&w=2

                      oh look, btrfs a lot faster than ext4 in a real world example.
                      Well that's borked thread. Follow https://lkml.org/lkml/2012/2/29/210

                      As one can notice, ext4 does not keep layout close thus requires more seeks to get the data read. Despite that it makes certain operations slower it also will eat your HDD(s) faster.

                      -df

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                      • #26
                        Check this out - https://lkml.org/lkml/2012/2/29/210

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                        • #27
                          Originally posted by energyman View Post
                          emm, no - the write cycles are not 'ok'. The smaller the structure the less write cycles you get. Modern flash chips write cycles are pathetic.
                          Sure. Do you have a source that shows a rough estimated lifetime of a Sandforce SSD? (normal use)

                          If it's 5 years than even halfing that to 2.5 years is definitely OK. As always, it depends on your usage scenario.

                          I think Anandtech has something but I can't find it at the moment.

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                          • #28
                            http://www.anandtech.com/show/5508/i...to-sandforce/7 - don't fill a complete Sandforce SSD with a compressed file system, or it's performance (and presumably lifetime) will decrease substantially.

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                            • #29
                              Originally posted by AnonymousCoward View Post
                              http://www.anandtech.com/show/5508/i...to-sandforce/7 - don't fill a complete Sandforce SSD with a compressed file system, or it's performance (and presumably lifetime) will decrease substantially.
                              Thanks. I can't wait for SSDs to become good and usable.

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                              • #30
                                Originally posted by AnonymousCoward View Post
                                http://www.anandtech.com/show/5508/i...to-sandforce/7 - don't fill a complete Sandforce SSD with a compressed file system, or it's performance (and presumably lifetime) will decrease substantially.
                                Hm, very interesting drawback of filesystem compression...

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