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SubjectRe: [PATCH v6] fat: Batched discard support for fat
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Hi again,

To Ogawa Hirofumi,

Do you still object to merge this feature for .40? As I said the
batched discard design is out-of-scope of this patch.
It's implemented at other batched discard, ext4, xfs and so on.

I hope fat is also support the batched discard.

Any opinions?

Thank you,
Kyungmin Park

On Thu, Mar 31, 2011 at 12:06 AM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Wednesday 30 March 2011, Kyungmin Park wrote:
>> On Wed, Mar 30, 2011 at 11:20 PM, Arnd Bergmann <arnd@arndb.de> wrote:
>> >> > If you just pass the minimum length, the file system could end up
>> >> > erasing a 4 MB section that spans two half erase blocks, or it
>> >> > could span a few clusters of the following erase block, both of
>> >> > which is not desirable from a performance point of view.
>> >>
>> >> Does those cards export such information correctly ?
>> >
>> > Most of the time, they export the erase block size correctly, but
>> > sometimes they lie. In fact, all of the cards I've seen report
>> > 4 MB erase blocks, but some cards in fact use 1.5 MB, 2 MB or 8 MB
>> > instead. I'm sure we will see 3 MB, 6 MB, 12 MB and 16 MB soon.
>> >
>> > SD cards do not export the page size, but most of them today use
>> > 16 KB. See my list at
>> >
>> > https://wiki.linaro.org/WorkingGroups/KernelConsolidation/Projects/FlashCardSurvey
>>
>> Thank you for your effort. but think it the original purpose of
>> batched discard feature.
>> Do you want to run the batched discard for SD card. I mean usually in
>> our environment SD card is optional.
>> and my goal is used for eMMC. and you know, eMMC and SD card internal
>> algorithm is different.
>
> I've done only a few measurements on eMMC, but for all I know, they
> use the same fundamental principles, just with a slightly different
> command set and physical interface. A number of manufacturers advertise
> controller chips that can be integrated on either MMC or SD packages,
> presumably using a different firmware.
>
>> The goal of SD card is performance but eMMC is different. it should
>> consider the reliability also. e.g., Sudden Power Off Recovery.
>
> I would have guessed the opposite, that sudden power off is much
> more important on SD cards that can simply be removed while writing.
>
> In practice, I would expect very little effort getting put into
> reliability on either of the two.
>
> Note that there was a patch on the mmc mailing list very recently
> to support the reliable write mode of eMMC.
>
> Also, when you want reliablility, you probably shouldn't use the
> FAT file system, since it lacks transactional writes. If you target
> eMMC media, there is usually no advantage in using FAT because the
> data does not get accessed by other operating systems.
>
>> > Good SSDs can deal with remapping 4k blocks, while cheap ones
>> > (SD cards, USB sticks, or low-end ATA SSDs) can only do operations
>> > on full blocks.
>> It really depends on devices. To increase the performance we have to
>> increase the channel and the way unit at internally.
>> Most NAND devices are similar there's no way except increase the
>> interleaving unit. and it affects the price.
>>
>> That's reason I like the batched discard. it's not easy to guess the
>> erase block at system level. so it's helpful scan the unused blocks
>> and try to trim at once.
>
> I'm absolutely in favor of batched discard, I just think we should
> apply some stricter rules to what blocks can get batched.
>
>> Please remember that the discard is just hint. If we write the flash
>> sequentially then we maybe don't need to trim it.
>
> With the current trends in NAND technology, the number of erase cycles
> keep going down, so I'd assume that we increasingly rely on discard
> in order to keep the wear levelling working, at least for the media
> that only do dynamic wear levelling. When you have a drive that supports
> static wear levelling, discard becomes more a performance optimization
> than a reliability requirement.
>
>        Arnd
>
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