Re: [PATCH] UBI: Decrease I/O count in ubi_scan

[Artem Bityutskiy <dedekind1@gmail.com>]

On 07/22/2009 11:51 AM, Corentin Chary wrote:
> On Tue, Jul 21, 2009 at 4:15 PM, Artem Bityutskiy<dedekind1@gmail.com>  wrote:
>> On 07/21/2009 04:42 PM, Corentin Chary wrote:
>>> [   12.544598] NAND device: Manufacturer ID: 0xec, Chip ID: 0x36
>>> (Samsung NAND 64MiB 1,8V 8-bit)
>>> [   12.683697] Scanni
>>> [   12.691395] 0x000000000000-0x000000800000 : "Linux Kernel"
>>> [   12.699687] 0x000000800000-0x000002800000 : "Filesystem"
>>> [   12.707648] 0x000002800000-0x000004000000 : "Free"
>>> [   12.719948] UBI: attaching mtd2 to ubi0
>>> [   12.728005] UBI: physical eraseblock size:   16384 bytes (16 KiB)
>>> [   12.741397] UBI: logical eraseblock size:    15360 bytes
>>> [   12.751721] UBI: smallest flash I/O unit:    512
>>> [   12.760202] UBI: VID header offset:          512 (aligned 512)
>>> [   12.767589] UBI: data offset:                1024
>>> [   13.047943] UBI: attached mtd2 to ubi0
>>> [   13.055450] UBI: MTD device name:            "Free"
>>> [   13.063343] UBI: MTD device size:            24 MiB
>>> [   13.071539] UBI: number of good PEBs:        1536
>>> [   13.084148] UBI: number of bad PEBs:         0
>>> [   13.095572] UBI: max. allowed volumes:       89
>>> [   13.105468] UBI: wear-leveling threshold:    4096
>>> [   13.113178] UBI: number of internal volumes: 1
>>> [   13.129826] UBI: number of user volumes:     1
>>> [   13.140834] UBI: available PEBs:             0
>>> [   13.153634] UBI: total number of reserved PEBs: 1536
>>> [   13.160852] UBI: number of PEBs reserved for bad PEB handling: 15
>>> [   13.167731] UBI: max/mean erase counter: 2/1
>>> [   13.177809] UBI: image sequence number: 0
>> This flash does not look like something which could fit a 200MiB
>> volume.
>>
>>>> And why do you presume is the improvement -
>>>> just less calls to MTD or really less I/O? Note MTD usually caches
>>>> the last read NAND page, so usually this is not about less I/O.
>>> In this case it seems vid and ec headers are not on the same page.
>> Strange. May be this is related to how they calculate ECC...
>> Or just a problem in the driver. Normally flashes like this would
>> have 256 byte sub-pages and have both headers in the same page.
>
> After reading more code, when HW ECC is enabled, atmel_nand only
> handle full page write.

OK, but I do not see why we have more I/O with your patch. It should
only be about doing less calls.

> A last question, to be sure,
> If we use something like (Samsung K9F2G08UQM) we have writesize = 2048
> and subpage = 512.
> VID header offset will be 512.
> ec_hdr is in the first subpage
> vid_hdr is in the second subpage
>
> So, there will two I/O (two subpage read) ?
> Reading nand_base.c I believe that the cache is only used when subpage
> read is not possible.

Yes, I think so. There was a patch from Alexey some time ago which
will make nand_base check ECCs only for the subpages you read, and
ignore the other ones.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)