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Re: ext3 / ext4 on USB flash drive?



Agh. This is great to have some of this clarified, Ted. It does
unfortunately reinorce my cynicism. But it also fills my heart. That
is, though it might be bad news to hear that I have most likely bought
a piece of crap, and that there's no way I can really tell what's
inside the cover unless either I do some kind of low-level alchemy of
a kind that mere users would normally be well advised to avoid, or the
manufacturer fesses up to its scam by publishing the specs; it is
nevertheless the case that this news has come from the filesystem
maintainer himself, and my faith in humanity is thus restored by this
simple proof of floss's egalitarian creed. Amen.

That said, the user forums are full of lost souls, despairing for want
of an answer to the simple question of whether they could well or
should not format their USB in ext3/4. Are you implying that it cannot
be known unless the manuf gives you the specs and if this is the case
and you've bought a bargain bucket flash drive (and assuming that
without a decent flash h/w controller it is true that ext3/4 will
thrash your drive) then you should stick to ext2? What then are the
features a USB flash spec you should look for as the minimum required
before installing ext4?

cheers

mb.

On 29 August 2013 16:46, Theodore Ts'o <tytso mit edu> wrote:
> On Thu, Aug 29, 2013 at 12:56:35PM +0100, Mark Ballard wrote:
>> I think this is really an attempt at user feedback, rather than user
>> discussion. But there's no such thing as a user-feedback mail list.
>>
>> Nevertheless, others may find this pertinent: why doesn't mke2fs
>> handle USB's competently? And if it does, why doesn't it reassure me
>> so? And how can I handle a linux-formatted USB flash drive in the
>> absence of my system giving me any guidance?
>
> The problem is that it really depends on the quality of the flash
> drive.   Two main things drive this:
>
> 1)  Whether the flash used in the drive is SLC. MLC, or TLC.  This
> will tell you how many write cycles the flash can support.  This is
> where you will see numbers such as 10,000 write cycles, all the way
> down to 1,000 write depending on the quality of the flash used.
>
> 2) The quality of the Flash Translation Layer (FTL).  The important
> thing to remember here is that not all parts of the storage device
> will get used to the same amount.  Some files will rarely change once
> they are installed on the device; others will change a huge amount.
> So a good Flash Translation Layer will spread out the wear so that if
> you have a 120GB device, with flash cells that support 10,000 write
> cycles, that ideally you can support up to 10,000 times 120GB == 120
> TB worth of writes.  Of course, that assumes 100% efficiency, which is
> unrealistic.  But some FTL's are incredibly bad, and in the worst
> case, if they aren't spreading the wear around at all, if you write a
> single block 1,000 times, and the flash only supports 1,000 write
> cycles, then that block will go bad.
>
> Unfortunately, the cheapest USB flash devices that you find in the
> bargain bin at the checkout counter of the Micro Center tend to use
> the crappiest flash media possible, and the crappist FTL's.  Some even
> will only have 512MB of flash even though it is advertised as having
> 8GB of flash, such that if you ever write more than 512MB worth of
> files on the flash, you will start losing data.  (Fortunately most
> people don't actually use the full capacity of their USB sticks, which
> is why the manufacturers can get away with this kind of fraud.)
>
> So this is fundamentally a problem with the quality of the hardware,
> and that's not something the file system can really compensate for.
> And there's no way to tell whether a particular USB device has has a
> high quality flash device, or is a craptastic flash device.  It's not
> like we can query the device for "I ripped off the purchaser" bit.  :-)
>
> Regards,
>
>                                         - Ted


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