Date: Wed, 2 Jul 2014 08:49:22 -0600 From: Warner Losh <imp@bsdimp.com> To: John-Mark Gurney <jmg@funkthat.com> Cc: freebsd-arm@freebsd.org Subject: Re: /tmp, /var/log, /var/tmp as /dev/md - why? Message-ID: <8756CB42-0B1B-4191-8A63-9D54FEE5D877@bsdimp.com> In-Reply-To: <20140702022042.GG45513@funkthat.com> References: <CADL2u4g65eo=7xxAt9j8JumyWneouhM2MGpcA9kfxJaCFWg95Q@mail.gmail.com> <201407011046.s61AkJpj006890@mech-cluster241.men.bris.ac.uk> <20140702022042.GG45513@funkthat.com>
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--Apple-Mail=_BCF575E8-B9A5-4E90-AF57-EAF27CE0393B Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=windows-1252 On Jul 1, 2014, at 8:20 PM, John-Mark Gurney <jmg@funkthat.com> wrote: > Anton Shterenlikht wrote this message on Tue, Jul 01, 2014 at 11:46 = +0100: >>> =46rom r.c.ladan@gmail.com Tue Jul 1 11:37:35 2014 >>>=20 >>> 2014-07-01 11:25 GMT+02:00 Anton Shterenlikht <mexas@bris.ac.uk>: >>>=20 >>>> Why is it a good idea to mount /tmp and some var dirs on memory = disks: >>>>=20 >>>> root@raspberry-pi:/usr/ports # df -m >>>> Filesystem 1M-blocks Used Avail Capacity Mounted on >>>> /dev/mmcsd0s2a 14694 777 12742 6% / >>>> devfs 0 0 0 100% /dev >>>> /dev/mmcsd0s1 16 3 13 20% /boot/msdos >>>> /dev/md0 28 4 22 16% /tmp >>>> /dev/md1 14 0 12 0% /var/log >>>> /dev/md2 4 0 4 0% /var/tmp >>>> root@raspberry-pi:/usr/ports # >>>>=20 >>>> Is this about speed or power, or maybe space? >>>>=20 >>>> Mostly write tear because you're using an SD card, and it improves = speed >>> too. >>=20 >> "write tear"? >> Is this a joke, or some technical term? >> I cannot find what it means. >=20 > it is a technical term, though I'd be surprised if any SD card had > an issue w/ that anymore=85 SD cards are made from NAND flash. NAND flash is different than a hard = disk in that it has only a limited number of times you can write to a = given spot before it becomes unusable (well, hard drives have this too, = but the numbers are huge (1e20 or something like that if memory serves). = This number can be as low as a few hundred for the really low-end cheap = crap parts, but typically is a few thousand. The reason for this is = because you have to erase each NAND page before you can program it, and = each time you erase it you expose the cell to a huge negative potential = voltage. This leads to trapping of electrons, as well as some minor = physical damage, usually along tiny flaws in the manufacturing process. = The net effect is that the dynamic range of the cell is reduced, which = leads to errors which eventually get bad enough that the pages become = useless. Since there=92s actual physical damage to each program/erase = cycle, it is referred to as =93wear and tear.=94 You=92ll see references = to =93wear leveling=94 which attempts to wear out each block at about = the same rate. > write tear is where when writing data, only part of the data gets > written and then you loose power... This is mostly an issue on flash > where you have to erase the data beforey ou can program it... Most > flash now have a layer of indirection so that they copy/write the > data to a new flash block, and then point the block there before > erasing the old data... (kinda like a log FS)=85 Yes, nearly all block devices based on flash have some kind of log-based = system under the covers. It is a shame they don=92t expose the log = directly, since a number of interesting optimizations could be made in = host software that are not possible going through the translation layer = a log-bsed system imposes on the card. But we get what we get... Warner --Apple-Mail=_BCF575E8-B9A5-4E90-AF57-EAF27CE0393B Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc Content-Description: Message signed with OpenPGP using GPGMail -----BEGIN PGP SIGNATURE----- Comment: GPGTools - https://gpgtools.org iQIcBAEBCgAGBQJTtBvyAAoJEGwc0Sh9sBEARMwP/jg2oo8B1pcn2rXvmkv8/FbJ 4Qcgr3VPrEoU6lKkbpBdbRPJN9lG+TSqZu0YT3ie6cG9bQ7A+Y5cc2a4zr+pav02 HeFsOU0IMsvZj1eePhaAS6Zz4naw3ZLG/9bE8sORCLMhXv2kKmyMg2Vi4PILq36m sMG1yRV+3ohP0YZ0enJwoAZ8h1HGrl/emOlXQgVL1+E2F5r8YGnWE0XgRc3n9qi2 lDGVVxQCRkLlTge8W+Fkh5lxqb1e0Tj4A8zpnhko/XkriFlM8nkn5vm6hfBHbqHW 9D61QxPoFnJa6sE0rB0wNpPk7D26njA0GoRKvNAcm6waBajlabZEDSPGif88oZv9 tshflFG4Z7AgCU03R2sv3w9fkKSR2NpAQOvjhMxUeBI26PXuY4KrnFu6aa8LeMVd /HjyM6kyUehOzCARGHDt1yGKAG9OSpQQqh8Fkai/kVY4OcGERPYf+OtAkqx25Ymj +cTihDVXlsVtO+OVAICqB6teeMWsZXOPEar1RmN+wCjHNstwdMnpt1Q7ud9kgxOW d/plyvwVkg0ulYvZa69sUrF8fe1XgErgLsW2ZX1wV968C5FNaCaoJB9df2a4M1ib BZA9TW2mtclzulMp5u4xcHCrqhL97sUkco7pE1K0EVJqOQtKNw4KSvENr/YrXb+k yPU18zre1L9bwvaUegon =igD5 -----END PGP SIGNATURE----- --Apple-Mail=_BCF575E8-B9A5-4E90-AF57-EAF27CE0393B--
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