Re: [PATCH 1/1] scsi: retrieve cache mode using ATA_16 if normalroutine fails

[Amit Sahrawat]

On Tue, Dec 13, 2011 at 10:24 AM, Amit Sahrawat
<amit.sahrawat83@xxxxxxxxx> wrote:
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> Hi James,
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> We verified the changes by actually putting the setup through a series of test cases. There were test cases where-in we were able to reduce the filesystem corruption by 100%.
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> Yes, In normal operation SYNC_CACHE gets called up when we do a safe removal of the device. But, Setting WCE also takes cares of sending the SYNC_CACHE command in between the operations also.
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> What we are making sure is, that by providing this interface we can have WCE enabled for some HDD –that will mark QUEUE ORDERING to QUEUE_ORDERED_DRAIN_FLUSH(Preflush-Postflush)(i.e., flushing will occur for all the Mass Storage devices - which in turn will keep on calling SYNC_CACHE for these devices.
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> We checked this on USB HDD’s, USB SSD, and USB Flash from a number of vendors – all seemed to work pretty well with these changes.
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> Please share your opinion.
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> Thanks & Regards,
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> Amit Sahrawat
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> On Tue, Dec 13, 2011 at 5:50 AM, Namjae Jeon <linkinjeon@xxxxxxxxx> wrote:
>>
>> 2011/12/12 James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>:
>> > On Mon, 2011-12-12 at 16:48 +0530, Amit Sahrawat wrote:
>> >> It has been observed that a number of USB HDD's do not respond correctly
>> >> to SCSI mode sense command(retrieve caching pages) which results in their
>> >> Write Cache being discarded by queue requests i.e., WCE if left set to
>> >> '0'(disabled).
>> >> This results in a number of Filesystem corruptions, when the device
>> >> is unplugged abruptly.
>> >
>> > Um, how would knowing the caching type correctly help?  If you surprise
>> > unplug the device, we can't send a flush to it anyway ...
>> Hi. James.
>> We can get device specification buffer to use ata_16 cmd.So we are
>> able to distinguish caching type by using 85byte of buffer.
>> And filesystem is using write barrier function to protect important
>> data like journaling data.
>> If filesystem is able to use write barrier by correctly setting WCE,
>> ordering can be guaranteed to flush data(preflush/postflush) to
>> internal write cache in hdd before power off. so filesystem can get
>> consistency and reliability by sudden plug and power off.
>> >
>> >> So, in order to identify the devices correctly - give it
>> >> a last try using ATA_16 after failure from normal routine.
>> >> Introduce a mechanism to store write-cache type using /sys/class/
>> >> interface, so that the normal code continues to function without errors.
>> >>
>> >> Signed-off-by: Amit Sahrawat <amit.sahrawat83@xxxxxxxxx>
>> >> Signed-off-by: Nam-Jae Jeon <namjae.jeon@xxxxxxxxxxx>
>> >
>> > This whole patch looks like a layering violation.  Why not just update
>> > the SAT layer to translate the MODE SENSE correctly?
>> Would plz you explain more ? I didn't clearly understand your point yet.
>> >
>> > James
>> >
>> >
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