lkml.org 
[lkml]   [2008]   [Oct]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: Filesystem for block devices using flash storage?
Jörn Engel wrote:
> On Wed, 8 October 2008 16:51:46 -0400, Chris Snook wrote:
>> Stefan Monnier wrote:
>>
>> Writes to magnetic disks are functionally atomic at the sector level. With
>> SSDs, writing requires an erase followed by rewriting the sectors that
>> aren't changing. This means that an ill-timed power loss can corrupt an
>> entire erase block, which could be up to 256k on some MLC flash. Unless
>
> What makes you think that? The standard mode of operation in El Cheapo
> devices is to write to a new eraseblock first, then delete the old one.
> An ill-timed power loss results in either the old or the new block being
> valid as a whole. This has been the standard ever since you could buy
> 4MB compactflash cards.
>
>> logfs tries to solve the write amplification problem by forcing all write
>> activity to be sequential. I'm not sure how mature it is.
>
> Still under development. What exactly do you mean by the write
> amplification problem?

Write amplification is where a 512 byte write turns into a 128k write,
due to erase block size.

>>> Or is there some hope for SSDs to provide access to the MTD layer in the
>>> not too distant future?
>> I hope not. The proper fix is to have the devices report their physical
>> topology via SCSI/ATA commands. This allows dumb software to function
>> correctly, albeit inefficiently, and allows smart software to optimize
>> itself. This technique also helps with RAID arrays, large-sector disks, etc.
>
> Having access to the actual flash would provide a large number of
> benefits. It just isn't a safe default choice at the moment.
>
>> I suspect that in the long run, the problem will go away. Erase blocks are
>> a relic of the days when flash was used primarily for low-power,
>> read-mostly applications. As the SSD market heats up, the flash vendors
>> will move to smaller erase blocks, possibly as small as the sector size.
>
> Do you have any information to back this claim? AFAICT smaller erase
> blocks would require more chip area per bit, making devices more
> expensive. If anything, I can see a trend towards bigger erase blocks.

Intel is claiming a write amplification factor of 1.1. Either they're
using very small erase blocks, or doing something very smart in the
controller.

-- Chris
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2008-10-13 19:33    [W:0.109 / U:0.244 seconds]
©2003-2011 Jasper Spaans. Advertise on this site