[linux-lvm] Directly using a logical volume

[linux-lvm] Directly using a logical volume

[Allen, Jack]

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        I have a system connected to a SAN via Fibre Channel interface.
The system sees 3 disk sdb, sdc and sdd. I put them in a volume group
and then allocated some logical volumes. If I use the logical volume to
read and write to directly for my application, if there is an error on a
write, I assume the write system call will return an error. Or does the
write give a good return value after putting the data in some system
buffer to be written later? Then is the write of the system buffer fails
later, my program would not know.

Thanks: 
        Jack Allen 


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Re: [linux-lvm] Directly using a logical volume

[Lars Ellenberg]

/ 2005-09-20 16:14:25 -0400
\ Allen, Jack:
>         I have a system connected to a SAN via Fibre Channel interface.
> The system sees 3 disk sdb, sdc and sdd. I put them in a volume group
> and then allocated some logical volumes. If I use the logical volume to
> read and write to directly for my application, if there is an error on a
> write, I assume the write system call will return an error. Or does the
> write give a good return value after putting the data in some system
> buffer to be written later? Then is the write of the system buffer fails
> later, my program would not know.

not exactly a linux-lvm question, is it?

man 2 write
man 2 fsync
man 3 open       (O_SYNC)

-- 
: Lars Ellenberg                                  Tel +43-1-8178292-0  :
: LINBIT Information Technologies GmbH            Fax +43-1-8178292-82 :
: Schoenbrunner Str. 244, A-1120 Vienna/Europe   http://www.linbit.com :

RE: [linux-lvm] Directly using a logical volume

[Allen, Jack]

-----Original Message-----
From: Lars Ellenberg [mailto:Lars.Ellenberg@linbit.com] 
Sent: Wednesday, September 21, 2005 8:58 AM
To: linux-lvm@redhat.com
Subject: Re: [linux-lvm] Directly using a logical volume


/ 2005-09-20 16:14:25 -0400
\ Allen, Jack:
>         I have a system connected to a SAN via Fibre Channel interface.
> The system sees 3 disk sdb, sdc and sdd. I put them in a volume group
> and then allocated some logical volumes. If I use the logical volume to
> read and write to directly for my application, if there is an error on a
> write, I assume the write system call will return an error. Or does the
> write give a good return value after putting the data in some system
> buffer to be written later? Then is the write of the system buffer fails
> later, my program would not know.

not exactly a linux-lvm question, is it?

man 2 write
man 2 fsync
man 3 open       (O_SYNC)

-- 
: Lars Ellenberg                                  Tel +43-1-8178292-0  :
: LINBIT Information Technologies GmbH            Fax +43-1-8178292-82 :
: Schoenbrunner Str. 244, A-1120 Vienna/Europe   http://www.linbit.com :

_______________________________________________

I have read the man pages and that would be "man 2 open". The O_SYNC seems
to only work for a regular file not a block device as the logical volume is.
This was part of the reason for my questions. On UNIX systems we use the
character device name and the O_SYNC does what you think it would there. I
know this is Linux and some things are different. Again the reason for my
questions. And yes it could be a linux-lvm question if the block device
presented by LVM works differently than a true SCSI block device. Again the
reason for the questions.

Thanks:
	Jack Allen

Re: [linux-lvm] Directly using a logical volume

[Lars Ellenberg]

/ 2005-09-21 13:23:40 -0400
\ Allen, Jack:
> From: Lars Ellenberg [mailto:Lars.Ellenberg@linbit.com] 
> Sent: Wednesday, September 21, 2005 8:58 AM
> / 2005-09-20 16:14:25 -0400
> \ Allen, Jack:
> >         I have a system connected to a SAN via Fibre Channel interface.
> > The system sees 3 disk sdb, sdc and sdd. I put them in a volume group
> > and then allocated some logical volumes. If I use the logical volume to
> > read and write to directly for my application, if there is an error on a
> > write, I assume the write system call will return an error. Or does the
> > write give a good return value after putting the data in some system
> > buffer to be written later? Then is the write of the system buffer fails
> > later, my program would not know.
> 
> not exactly a linux-lvm question, is it?
> 
> man 2 write
> man 2 fsync
> man 3 open       (O_SYNC)

> I have read the man pages and that would be "man 2 open". The O_SYNC seems

man 3 open happened to be the open (POSIX) man page here.

> to only work for a regular file not a block device as the logical volume is.

to the best of my knowledge, the O_SYNC works on block devices, too.

> This was part of the reason for my questions. On UNIX systems we use the
> character device name and the O_SYNC does what you think it would there.

there is also O_DIRECT, which might be more what you want to use.
and there is the "raw" interface, too
(which is deprecated, afaik, at least in 2.6 kernels, where you should
use O_DIRECT instead).

> I
> know this is Linux and some things are different. Again the reason for my
> questions. And yes it could be a linux-lvm question if the block device
> presented by LVM works differently than a true SCSI block device. Again the
> reason for the questions.

did not mean to offend you.
this is all abstracted in the generic block device layer,
so it does not make a difference whether it is ide/sata/scsi/lvm.

I'd recommend you use O_DIRECT|O_SYNC, then.
note that this implies buffer allignment restrictions.

this should just work as described by you.
still it probably does not hurt to have fsync in the code where appropriate.

cheers,

-- 
: Lars Ellenberg                                  Tel +43-1-8178292-0  :
: LINBIT Information Technologies GmbH            Fax +43-1-8178292-82 :
: Schoenbrunner Str. 244, A-1120 Vienna/Europe   http://www.linbit.com :