Messages in this thread | | | Date | Tue, 01 Apr 2008 11:39:37 +0200 | From | Tomasz Chmielewski <> | Subject | Re: UBIFS vs Logfs (was [RFC PATCH] UBIFS - new flash file system) |
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Artem Bityutskiy schrieb:
> Tomasz Chmielewski wrote: >> For me, the motivators to wait for LogFS are mainly the facts that it >> can work on traditional block devices, and not only on pure flash: > > Sorry Thomasz, for me this makes zero sense. There are _much_ better file > systems for block devices.
Such as? Flash (also on block devices) is slow and expensive (when compared to modern hard disks) and therefore compression is *very* useful here. Not only it can potentially save you money; reads and writes will be smaller/faster (unless you're editing music and videos, where one won't use flash anyway), flash will wear out slower etc.
Is there a filesystem for Linux which can provide transparent compression and works for block devices (other than user-space NTFS or some outdated ext2 patches)? I don't think so.
> UBIFS may work on top of a block device as > well (just needs few hacks to make it possible) - it is not a problem > at all, it is just _senseless_.
Do you mean using hacks like block2mtd? It's hacky, and pretty hard to boot a system this way (need to build own initramfs, with a static block2mtd or loads of libraries - not something an average user would like to do; no distro supports it; updating a kernel would be a pain etc.).
> JFFS2/UBIFS/LogFS is a separate _class_ of file-systems. The are designed > for _flash_, which has completely different work model then block device. > They are _native_ flash file systems. > Here are more details: > http://www.linux-mtd.infradead.org/faq/general.html#L_mtd_vs_hdd
It's a good comparison, but it doesn't show disadvantages of using traditional filesystems on flash-based block devices.
I just mentioned the reasons why a filesystem like LogFS would be useful on block devices: there are valid reasons to do it.
(...)
> The whole _point_ of this separate class of FSes is because we believe > we may do much _better_ job if we use flash _natively_, instead of using > FTL. FTL is the place where you loose performance, reliability, and so on.
True. Unfortunately, there is no way to access flash directly on flash-based block devices (USB-sticks, IDE-flash disks, SSD disks etc.).
Therefore, could an answer be: use a traditional filesystem?
Unfortunately, traditional filesystems were rather designed for rotating media / cheap disks (no transparent compression; tend to accumulate writes in one area of the disk - more on that - below).
> And you are saying about using a native flash FS on top of a block device > like an SD card. This is just not sane: SD card first emulates a block > device > for you, looses performance at this point, then you again emulate a flash > on top of this, and suffer from this again.
Performance is only one factor in the equation. Other factors are: cost and reliability.
I speak from experience: flash-based block devices tend to have poor wear-levelling (at least Transcend IDE-flash disks). To reproduce: - format a 2 GB Transcend IDE-flash disk with ext3 - write a small file (50-100 kB) - update that file ~several hundred thousand times - as you finish, IDE-flash disk will have 200-300 badblocks
If wear-levelling on the underlying IDE-flash device was any decent, writes would be spread on the whole ~2GB surface, totalling in many more successful writes.
Again: this is my experience, although it may contradict the theory underlined in the link you gave earlier. You have much more experience with flash file systems: correct me where I'm wrong.
-- Tomasz Chmielewski http://wpkg.org
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