FILE_FLAG_WRITE_THROUGH

FILE_FLAG_WRITE_THROUGH

[Rebelde]

Hi

On Windows XP I can write directly to disk whitout cache write using the 
CreateFile function with FILE_FLAG_WRITE_THROUGH. Could someone tell me 
how to make this on Linux?
Thank you for improve and sorry for my poor english.

Reinaldo
São Paulo - SP - Brasil
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Re: FILE_FLAG_WRITE_THROUGH

[Steve Graegert]

On 4/4/06, Rebelde <jrsilva2k@gmail.com> wrote:
> Hi
>
> On Windows XP I can write directly to disk whitout cache write using the
> CreateFile function with FILE_FLAG_WRITE_THROUGH. Could someone tell me
> how to make this on Linux?

What you're looking for is a filesystem-specific feature and can be
utilized using the O_DIRECT flag in the open(2) system call.  If
you're filesystem does not support raw i/o EINVAL is returned.  Since
I am using XFS I can't tell you about other fs, but I suppose Ext2,
Ext3 and JFS do support it. (Corrections welcome.)

	\Steve

Re: FILE_FLAG_WRITE_THROUGH

[Glynn Clements]

Rebelde wrote:

> On Windows XP I can write directly to disk whitout cache write using the 
> CreateFile function with FILE_FLAG_WRITE_THROUGH. Could someone tell me 
> how to make this on Linux?

Pass the O_SYNC flag to open(). This will cause write() calls to block
until the data has been sent to the drive.

The O_DIRECT flag suggested by Steve is probably overkill. It requires
that the buffer start address, buffer size and file offset are all
multiples of the filesystem's block size, and only works on some
filesystems.

-- 
Glynn Clements <glynn@gclements.plus.com>

Re: FILE_FLAG_WRITE_THROUGH

[Progga]

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On Wed, Apr 05, 2006 at 05:02:20AM +0100, Glynn Clements wrote:

> The O_DIRECT flag suggested by Steve is probably overkill. It requires
> that the buffer start address, buffer size and file offset are all
> multiples of the filesystem's block size, and only works on some
> filesystems.

Although it works for a single file, how good is fsync() in this case?


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Re: FILE_FLAG_WRITE_THROUGH

[Steve Graegert]

On 4/5/06, Progga <abulfazl@juniv.edu> wrote:
> On Wed, Apr 05, 2006 at 05:02:20AM +0100, Glynn Clements wrote:
>
> > The O_DIRECT flag suggested by Steve is probably overkill. It requires
> > that the buffer start address, buffer size and file offset are all
> > multiples of the filesystem's block size, and only works on some
> > filesystems.
>
> Although it works for a single file, how good is fsync() in this case?

fsync(2) does not ensure that all data has actually been written to
disk.  The controller may indicate that all data is stable, but it
does so even if it is still in its internal cache.  From this point of
view, fsync(2) is not a replacement for O_DIRECT.

	\Steve

Re: FILE_FLAG_WRITE_THROUGH

[Glynn Clements]

Steve Graegert wrote:

> > > The O_DIRECT flag suggested by Steve is probably overkill. It requires
> > > that the buffer start address, buffer size and file offset are all
> > > multiples of the filesystem's block size, and only works on some
> > > filesystems.
> >
> > Although it works for a single file, how good is fsync() in this case?
> 
> fsync(2) does not ensure that all data has actually been written to
> disk.  The controller may indicate that all data is stable, but it
> does so even if it is still in its internal cache.  From this point of
> view, fsync(2) is not a replacement for O_DIRECT.

If the drive doesn't accurately indicate whether data has been written
to the physical medium, that will affect everything, including
O_DIRECT. There have been cases of drives which indicate that data has
been written even when it's still in the cache, in order to improve
benchmark scores.

I don't believe that there's any difference between O_SYNC, O_DIRECT
or fsync() in terms of their interpretation of "written to the
hardware"; they all send a "flush" command to the drive and block
until the drive reports completion.

The advantage of O_DIRECT is that it won't displace existing blocks
from the kernel's buffer cache, which might be useful if you're
writing a lot of data and won't be reading it back in any time soon.

According to:

	http://support.microsoft.com/default.aspx?scid=kb%3Ben-us%3B99794

that is more than what FILE_FLAG_WRITE_THROUGH does:

	The FILE_FLAG_WRITE_THROUGH flag for CreateFile() causes any
	writes made to that handle to be written directly to the file
	without being buffered. The data is cached (stored in the disk
	cache); however, it is still written directly to the file. 
	This method allows a read operation on that data to satisfy
	the read request from cached data (if it's still there),
	rather than having to do a file read to get the data. The
	write call doesn't return until the data is written to the
	file. This applies to remote writes as well--the network
	redirector passes the FILE_FLAG_WRITE_THROUGH flag to the
	server so that the server knows not to satisfy the write
	request until the data is written to the file.

O_DIRECT appears closer to FILE_FLAG_NO_BUFFERING:

	The FILE_FLAG_NO_BUFFERING takes this concept one step further
	and eliminates all read-ahead file buffering and disk caching
	as well, so that all reads are guaranteed to come from the
	file and not from any system buffer or disk cache. When using
	FILE_FLAG_NO_BUFFERING, disk reads and writes must be done on
	sector boundaries, and buffer addresses must be aligned on
	disk sector boundaries in memory.

IOW, FILE_FLAG_WRITE_THROUGH corresponds to O_SYNC while
FILE_FLAG_NO_BUFFERING corresponds to O_DIRECT.

-- 
Glynn Clements <glynn@gclements.plus.com>

Re: FILE_FLAG_WRITE_THROUGH

[Steve Graegert]

On 4/5/06, Glynn Clements <glynn@gclements.plus.com> wrote:
>
> Steve Graegert wrote:
>
> > > > The O_DIRECT flag suggested by Steve is probably overkill. It requires
> > > > that the buffer start address, buffer size and file offset are all
> > > > multiples of the filesystem's block size, and only works on some
> > > > filesystems.
> > >
> > > Although it works for a single file, how good is fsync() in this case?
> >
> > fsync(2) does not ensure that all data has actually been written to
> > disk.  The controller may indicate that all data is stable, but it
> > does so even if it is still in its internal cache.  From this point of
> > view, fsync(2) is not a replacement for O_DIRECT.
>
> If the drive doesn't accurately indicate whether data has been written
> to the physical medium, that will affect everything, including
> O_DIRECT. There have been cases of drives which indicate that data has
> been written even when it's still in the cache, in order to improve
> benchmark scores.

That's what all (IDE) drives do; they simply lie about the state of
data in their internal caches.  And the kernel believes them; it has
no idea of what's in the drive's cache and has no controll over it. 
SCSI drives also do caching, but the SCSI layer uses tags to turn
caching on and off if necessary.  This is exactly what happens with
O_DIRECT on Linux.

> I don't believe that there's any difference between O_SYNC, O_DIRECT
> or fsync() in terms of their interpretation of "written to the
> hardware"; they all send a "flush" command to the drive and block
> until the drive reports completion.

It depends heavily on the i/o subsystem and the i/o media.  There is a
significant difference between IDE and SCSI drives.  The latter report
data to be stable on disk only, if the blocks held in cache have found
their place on the disk.

> The advantage of O_DIRECT is that it won't displace existing blocks
> from the kernel's buffer cache, which might be useful if you're
> writing a lot of data and won't be reading it back in any time soon.

Exactly, and most SCSI (and some more advanced IDE/SATA controllers)
take that into account.  They offer fairly good services for drivers
to allow OSes to handle some special i/o requests as it is the case
for O_DIRECT.

> According to:
>
>         http://support.microsoft.com/default.aspx?scid=kb%3Ben-us%3B99794
>
> that is more than what FILE_FLAG_WRITE_THROUGH does:
>
>         The FILE_FLAG_WRITE_THROUGH flag for CreateFile() causes any
>         writes made to that handle to be written directly to the file
>         without being buffered. The data is cached (stored in the disk
>         cache); however, it is still written directly to the file.
>         This method allows a read operation on that data to satisfy
>         the read request from cached data (if it's still there),
>         rather than having to do a file read to get the data. The
>         write call doesn't return until the data is written to the
>         file. This applies to remote writes as well--the network
>         redirector passes the FILE_FLAG_WRITE_THROUGH flag to the
>         server so that the server knows not to satisfy the write
>         request until the data is written to the file.
>
> O_DIRECT appears closer to FILE_FLAG_NO_BUFFERING:
>
>         The FILE_FLAG_NO_BUFFERING takes this concept one step further
>         and eliminates all read-ahead file buffering and disk caching
>         as well, so that all reads are guaranteed to come from the
>         file and not from any system buffer or disk cache. When using
>         FILE_FLAG_NO_BUFFERING, disk reads and writes must be done on
>         sector boundaries, and buffer addresses must be aligned on
>         disk sector boundaries in memory.
>
> IOW, FILE_FLAG_WRITE_THROUGH corresponds to O_SYNC while
> FILE_FLAG_NO_BUFFERING corresponds to O_DIRECT.

Agree on that one.  I read the OP as a request on how to do "true"
cacheless i/o.
 Nevertheless, it should be noted that O_DIRECT usually downgrades
overall performance if called synchronously.  There is no point in
doing so, exactly because of the above reasons.

	\Steve

Re: FILE_FLAG_WRITE_THROUGH

[Steve Graegert]

On 4/5/06, Glynn Clements <glynn@gclements.plus.com> wrote:
>
> Rebelde wrote:
>
> > On Windows XP I can write directly to disk whitout cache write using the
> > CreateFile function with FILE_FLAG_WRITE_THROUGH. Could someone tell me
> > how to make this on Linux?
>
> Pass the O_SYNC flag to open(). This will cause write() calls to block
> until the data has been sent to the drive.
>
> The O_DIRECT flag suggested by Steve is probably overkill. It requires
> that the buffer start address, buffer size and file offset are all
> multiples of the filesystem's block size, and only works on some
> filesystems.

Well, the O_SYNC flag causes, i.e. write(2), to block until the data
has been physically written to disk, but it does not prevent the C
library/kernel from caching the bytes.  O_DIRECT, on the other hand,
circumvents the caching mechanisms and allows for raw access, although
it may degrade performance.  While sematically difficult, I think this
technique is nearest equivalent to the FILE_FLAG_WRITE_THROUGH flag
known from Windows.

	\Steve

Re: FILE_FLAG_WRITE_THROUGH

[Al Boldi]

Steve Graegert wrote:
> On 4/5/06, Glynn Clements <glynn@gclements.plus.com> wrote:
> > Rebelde wrote:
> > > On Windows XP I can write directly to disk whitout cache write using
> > > the CreateFile function with FILE_FLAG_WRITE_THROUGH. Could someone
> > > tell me how to make this on Linux?
> >
> > Pass the O_SYNC flag to open(). This will cause write() calls to block
> > until the data has been sent to the drive.
> >
> > The O_DIRECT flag suggested by Steve is probably overkill. It requires
> > that the buffer start address, buffer size and file offset are all
> > multiples of the filesystem's block size, and only works on some
> > filesystems.
>
> Well, the O_SYNC flag causes, i.e. write(2), to block until the data
> has been physically written to disk, but it does not prevent the C
> library/kernel from caching the bytes.  O_DIRECT, on the other hand,
> circumvents the caching mechanisms and allows for raw access, although
> it may degrade performance.

Or increase performance because dcache costs dear cpu-cycles?

Thanks!

--
Al