[Gluster-devel] lseek

[Xavier Hernandez]

Hello Ian,

lseek calls are handled internally by the kernel and they never reach 
the user land for fuse calls. lseek only updates the current file offset 
that is stored inside the kernel file's structure. This value is what is 
passed to read/write fuse calls as an absolute offset.

There isn't any problem in this behavior as long as you hide all size 
manipulations from fuse. If you write a translator that compresses a 
file, you should do so in a transparent manner. This means, basically, that:

1. Whenever you are asked to return the file size, you must return the 
size of the uncompressed file
2. Whenever you receive an offset, you must translate that offset to the 
corresponding offset in the compressed file and work with that
3. Whenever you are asked to read or write data, you must return the 
number of uncompressed bytes read or written (even if you have 
compressed the chunk of data to a smaller size and you have physically 
written less bytes).
4. All read requests must return uncompressed data (this seems obvious 
though)

This guarantees that your manipulations are not seen in any way by any 
upper translator or even fuse, thus everything should work smoothly.

If you respect these rules, lseek (and your translator) will work as 
expected.

In particular, when a user calls lseek with SEEK_END, the kernel takes 
the size of the file from the internal kernel inode's structure. This 
size is obtained through a previous call to lookup or updated using the 
result of write operations. If you respect points 1 and 3, this value 
will be correct.

In gluster there are a lot of fops that return a iatt structure. You 
must guarantee that all these functions return the correct size of the 
file in the field ia_size to be sure that everything works as expected.

Xavi

On 05/14/2012 11:51 AM, Ian Latter wrote:
> Hello Xavi,
>
>
>    Ok - thanks.  I was hoping that this was how read
> and write were working (i.e. with absolute offsets
> and not just getting relative offsets from the current
> seek point), however what of the raw seek
> command?
>
>       len = lseek(fd, 0, SEEK_END);
>
>       Upon  successful completion, lseek() returns
>       the resulting offset location as measured in
>       bytes from the beginning of the  file.
>
>    Any idea on where the return value comes from?
> I will need to fake up a file size for this command ..
>
>
>
> ----- Original Message -----
>> From: "Xavier Hernandez"<xhernandez at datalab.es>
>> To:<gluster-devel at nongnu.org>
>> Subject:  Re: [Gluster-devel] lseek
>> Date: Mon, 14 May 2012 09:48:17 +0200
>>
>> Hello Ian,
>>
>> there is no such thing as an explicit seek in glusterfs.
> Each readv,
>> writev, (f)truncate and rchecksum have an offset parameter
> that tells
>> you the position where the operation must be performed.
>>
>> If you make something that changes the size of the file
> you must make it
>> in a way that it is transparent to upper translators. This
> means that
>> all offsets you will receive are "real" (in your case,
> offsets in the
>> uncompressed version of the file). You should calculate in
> some way the
>> equivalent offset in the compressed version of the file
> and send it to
>> the correspoding fop of the lower translators.
>>
>> In the same way, you must return in all iatt structures
> the real size of
>> the file (not the compressed size).
>>
>> I'm not sure what is the intended use of NONSEEKABLE, but
> I think it is
>> for special file types, like devices or similar that are
> sequential in
>> nature. Anyway, this is a fuse flag that you can't return
> from a regular
>> translator open fop.
>>
>> Xavi
>>
>> On 05/14/2012 03:22 AM, Ian Latter wrote:
>>> Hello,
>>>
>>>
>>>     I'm looking for a seek (lseek) implementation in
>>> one of the modules and I can't see one.
>>>
>>>     Do I need to care about seeking if my module
>>> changes the file size (i.e. compresses) in Gluster?
>>> I would have thought that I did except that I believe
>>> that what I'm reading is that Gluster returns a
>>> NONSEEKABLE flag on file open (fuse_kernel.h at
>>> line 149).  Does this mitigate the need to correct
>>> the user seeks?
>>>
>>>
>>> Cheers,
>>>
>>>
>>>
>>> --
>>> Ian Latter
>>> Late night coder ..
>>> http://midnightcode.org/
>>>
>>> _______________________________________________
>>> Gluster-devel mailing list
>>> Gluster-devel at nongnu.org
>>> https://lists.nongnu.org/mailman/listinfo/gluster-devel
>> _______________________________________________
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>>
>
> --
> Ian Latter
> Late night coder ..
> http://midnightcode.org/
>
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