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Barriers and journaling filesystems

Barriers and journaling filesystems

Posted May 22, 2008 12:44 UTC (Thu) by joern (subscriber, #22392)
In reply to: Barriers and journaling filesystems by ekj
Parent article: Barriers and journaling filesystems

> Typical flash-memory today is rated for 1M writes. There are internal wear-leveling that
ensures that this is that number of writes to the ENTIRE module. (i.e. it is impossible to
wear out flash faster by writing repeatedly to the same location)

This happens to be wrong on both accounts.  Typical flash-memory today is rated at either 10k
or 100k - the lower number being for MLC flashes, which are cheaper and therefore can be
expected in your average cheap medium from Fry's.

Far worse, the normal wear leveling scheme does _not_ cover the complete device.  It covers
some part of it, typically 16M or so.  The next part is also wear-leveled in itself, but not
wrt. any other part of the device.  Therefore having a really hot area like a 32M journal is
comparable to disabling the wear leveling for the device completely.  After 10k journal wraps
you're depending on pure luck.

The horse may be locked away in a broken shed, but it's still kicking.

[ To be fair, some expensive devices are far far better.  Some expensive devices are just that
- more expensive.  So do your own QA to be sure. ]


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Barriers and journaling filesystems

Posted May 22, 2008 15:13 UTC (Thu) by drag (subscriber, #31333) [Link]

> This happens to be wrong on both accounts.  Typical flash-memory today is rated at either
10k or 100k - the lower number being for MLC flashes, which are cheaper and therefore can be
expected in your average cheap medium from Fry's.


Well you wouldn't buy the cheapest thing you can find when you go want to use it in a server,
right? So you make sure you get the 'high endurance' versions of the drives with SLC NAND
chips and make sure that you go through a cycle of swapping it out every year or so.

The thing is is that people are actually using flash to help speed up disk access in their
datacenters.  This isn't the first time I've heard of people doing this sort of thing.

Personally I work with a lot of flash media. The cheaper stuff. I haven't been here long, but
I've talked to people that have been working here for 20 years. (of course they haven't been
using CF cards that long). Nobody has yet to see any sort of flash media failing that they can
remember. The actual physical connections (the plastic holes for the pins get malformed) get
all screwed up before the any actual data ever gets corrupted. 


Were do you get your numbers for the 16M wear leveling? Typically your dealing with media that
is at least 512 megs and soon you'll have a hard time finding new stuff that is under 2 gigs.
I am doubtful that only 16megs out of 2gigs is going to be wear leveled.

Barriers and journaling filesystems

Posted May 22, 2008 15:24 UTC (Thu) by joern (subscriber, #22392) [Link]

> Personally I work with a lot of flash media. The cheaper stuff. I haven't been here long,
but I've talked to people that have been working here for 20 years. (of course they haven't
been using CF cards that long). Nobody has yet to see any sort of flash media failing that
they can remember. The actual physical connections (the plastic holes for the pins get
malformed) get all screwed up before the any actual data ever gets corrupted.

I know of reports.  Ext3 on CF seems to be fairly good at corrupting stuff, particularly data
that is stored close to the journal.  Whether the cards in question used SLC or MLC I don't
know.  The pesky thing about them is that vendors hardly ever publish information at all.

> Were do you get your numbers for the 16M wear leveling? Typically your dealing with media
that is at least 512 megs and soon you'll have a hard time finding new stuff that is under 2
gigs. I am doubtful that only 16megs out of 2gigs is going to be wear leveled.

http://www.linuxconf.eu/2007/papers/Engel.pdf
Mainly based on the smartmedia spec and some reverse engineering.  I didn't do the latter
myself, though.

Barriers and journaling filesystems

Posted May 23, 2008 2:21 UTC (Fri) by drag (subscriber, #31333) [Link]

> Whether the cards in question used SLC or MLC I don't know.  The pesky thing about them is
that vendors hardly ever publish information at all.


Ya. There is only a handful of people that actually make flash media. Maybe four companies in
total, I forget. 

Everybody that sells that flash media to end users uses a hodgepodge of different sources for
different products. Cheaper folks will mix in different flashes for different  production runs
on the same product... We ran into this problem with Kingston shipping devices that had any
sizes from 470megs on up for their 512 meg products.

So I'd stay far away from vendors that don't publish details about their products for anything
serious. 

Barriers and journaling filesystems

Posted Jun 12, 2008 14:42 UTC (Thu) by salimma (subscriber, #34460) [Link]

Not commenting on whether the 16MB information is correct or not, but grandparent's point is
not that only 16MB gets write-leveled; it's that for the purpose of write-leveling, the drive
is treated as a series of 16MB blocks, each of them are write-leveling within themselves.

(The write-leveling circuitry would then be much simpler -- imagine a parallel series of, say,
8-bit adders, compared to a 64-bit adders made up of 8-bit adders)


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