Re: Re: [PATCH V2 0/2] Auto stop async-write on block device when device removed.

From: majianpeng
Date: Tue Sep 24 2013 - 21:33:09 EST

>majianpeng <majianpeng@xxxxxxxxx> writes:
>>>majianpeng <majianpeng@xxxxxxxxx> writes:
>>>> For async-write on block device,if device removed,but the vfs don't know it.
>>>> It will continue to do.
>>>> Patch1 set size of inode of block device to zero when removed disk.By this,vfs know
>>>> disk changed.
>>>> Path2 add size-check on blk_aio_write.If pos of write larger than size of inode,it will
>>>> return zero.So the user can check disk state.
>>>OK, so the basic problem is that __generic_file_aio_write will always
>>>return 0 after device removal, yes? I'm not sure why that's a real
>>>issue, can you explain exactly why you're trying to change this?
>> At prenset, the __generic_file_aio_write don't return zero rather that the wanted size.
>> So the user can't know the disk removed.
>> For example:
>> dd if=/dev/zero of=usb-disk bs=64k
>> When removed usb-disk, dd stoped until reached the endof usb-disk.
>Ah, right, it's just writing to the page cache. I think the only reason
>you get more timely errors when doing the same thing to a file on a file
>system is that there is some synchronous metadata or journal I/O that
>will get EIO and result in the file system being set read-only.
>The bigger question is whether we want to change this long-standing
>behaviour of how our write-back cache works. I don't know that it's
>really worth it, honestly. If you want to ensure data is on disk, you
>open the file O_SYNC or you issue an fsync, and those calls will return
>an error for a removed block device. So, I guess I'll ask the same
>question again: why are you looking at this? Is there some application
>you care about that does buffered I/O to the block device and never does
>an fsync?
Yes, for my company, we used our filesystem in userspace on block-device.
For the performance, we used buffer-wrtite not sync-write.
For my workload, we allow user to remove disk whether disk working or not.
Now, we check the state of disk from /proc/partitions at the same interval.

This patchset don't change write-back cache works.It only let vfs know the state of lower-device.
I think it make a sense.

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