Hi All, I want to wait in my code till the IO actually goes to the disk. For example struct buffer_head *bh=sb_bread(sb, 20) strcpy(bh->b_data, "Some Data"); /* mark and sync */ mark_buffer_dirty(bh); sync_dirty_buffer(bh); The above code only adds the bh to the request queue and returns. I need something write_buffer_to_disk(bh) Which will return only if the data has been written on disk. Please help -- Regards, Rishi Agrawal
On Fri, Mar 28, 2014 at 11:16:30PM +0530, Rishi Agrawal wrote:
Hi All,
I want to wait in my code till the IO actually goes to the disk.
That's an issue, as you really don't know what a "disk" is from within the kernel :) Back up, what exactly are you trying to do?
For example
struct buffer_head *bh=sb_bread(sb, 20)
strcpy(bh->b_data, "Some Data");
/* mark and sync */ mark_buffer_dirty(bh); sync_dirty_buffer(bh);
The above code only adds the bh to the request queue and returns.
And that's all you should need. Why do you feel you need more? thanks, greg k-h
Hi Greg, On Fri, Mar 28, 2014 at 11:31 PM, Greg KH <greg@kroah.com> wrote:
On Fri, Mar 28, 2014 at 11:16:30PM +0530, Rishi Agrawal wrote:
Hi All,
I want to wait in my code till the IO actually goes to the disk.
That's an issue, as you really don't know what a "disk" is from within the kernel :)
Back up, what exactly are you trying to do?
Its a virtual machine, so no worries here, more over I am using a loop device.
For example
struct buffer_head *bh=sb_bread(sb, 20)
strcpy(bh->b_data, "Some Data");
/* mark and sync */ mark_buffer_dirty(bh); sync_dirty_buffer(bh);
The above code only adds the bh to the request queue and returns.
And that's all you should need. Why do you feel you need more?
Actually I am developing a kernel module through which I am writing to the device. I am not able to the written data in some cases, the code path is same everywhere. So I thought maybe its not getting flushed on the disk and I am expecting the write on the disk too early.
I want to wait till the write completes as performance is not I am worried about right now.
thanks,
greg k-h
-- Regards, Rishi Agrawal
2014-03-28 23:16 GMT+05:30 Rishi Agrawal <rishi.b.agrawal@gmail.com>:
Hi All,
I want to wait in my code till the IO actually goes to the disk.
For example
struct buffer_head *bh=sb_bread(sb, 20)
strcpy(bh->b_data, "Some Data");
/* mark and sync */ mark_buffer_dirty(bh); sync_dirty_buffer(bh);
The above code only adds the bh to the request queue and returns.
My memory is very rusty. So I may be wrong. Look for what sync_filesystem does. However, there is no guarantee that the data will be actually written to disk. I have heard instances where a caching layer in the disk tells the filesystem that the data is written but the data was not written and there was a power failure and an ensuing loss of data. So from the filesystem side, you can only provide a somewhat unreliable confirmation only most of the times.
I need something
write_buffer_to_disk(bh)
Which will return only if the data has been written on disk.
Please help --
Regards, Rishi Agrawal
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
-- Sankar P http://psankar.blogspot.com
On Sat, 29 Mar 2014 18:17:52 +0530, Sankar P said:
However, there is no guarantee that the data will be actually written to disk. I have heard instances where a caching layer in the disk tells the filesystem that the data is written but the data was not written and there was a power failure and an ensuing loss of data.
Actually, let me go a step further - I know of no current disks which support write caching that *don't* lie to the OS and say "I/O complete, data is on the disk" when it lands in the cache (leaving you vulnerable if there's a power hit before it flushes the cashe). Even more evil - although you'd *think* that the solution is to just disable the write cache so the disk can't lie and has to wait for the data to hit the platters, that's not quite true. There's been disks (especially on the very low end where they Just Don't Care, and on the high end were numbers are everything) that will *say* they disabled the write cache, but it's still doing it inside....
Thanks all, On Sun, Mar 30, 2014 at 9:59 PM, <Valdis.Kletnieks@vt.edu> wrote:
On Sat, 29 Mar 2014 18:17:52 +0530, Sankar P said:
However, there is no guarantee that the data will be actually written to disk. I have heard instances where a caching layer in the disk tells the filesystem that the data is written but the data was not written and there was a power failure and an ensuing loss of data.
Actually, let me go a step further - I know of no current disks which support write caching that *don't* lie to the OS and say "I/O complete, data is on the disk" when it lands in the cache (leaving you vulnerable if there's a power hit before it flushes the cashe).
Even more evil - although you'd *think* that the solution is to just disable the write cache so the disk can't lie and has to wait for the data to hit the platters, that's not quite true. There's been disks (especially on the very low end where they Just Don't Care, and on the high end were numbers are everything) that will *say* they disabled the write cache, but it's still doing it inside....
Though I can't be still sure of that the data has been flushed to disk something like wait_on_buffer() will help me a bit I am still curious to know what happens in journalling, as the journal NEEDS to be committed to the disk, before data writes/modifcation proceed. Same for Direct IO as Vineet Suggested I could not look at the code, will look the code. -- Regards, Rishi Agrawal
participants (4)
-
Greg KH -
Rishi Agrawal -
Sankar P -
Valdis.Kletnieks＠vt.edu