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IDS Forum

RE: Is Logical Volume a raw device on HPUX?

Posted By: ART KAGEL, BLOOMBERG/ 65E 55TH
Date: Tuesday, 26 August 2003, at 10:55 a.m.

In Response To: Re: Is Logical Volume a raw device on HPUX? (ART KAGEL, BLOOMBERG/ 65E 55TH)

----- Original Message -----
From: Thompson Richard His-Herts
At: 8/26 9:41
>
> FYI
>
> A cooked file is not the block special file that you assume but a straight
> unix file rather than a lv or raw disk, it is cooked per se 'cos INFORMIX
> assumes that it has direct access to all files (special or otherwise). The
> delays you experience using the block special file are to do with the
> syncing of the block/buffer to disk by the OS rather than letting INFORMIX
> have direct access to the raw disk.

No! I specifically make distinction between COOKED filesystem files and COOKED
(ie block) devices. BOTH are COOKED in the sense that you specify. First BOTH
are opened by IDS in O_SYNC mode so that writes are synced immediately to disk,
however, as I stated, unlike RAW or character devices BOTH types of COOKED
chunks must transfer the data to OS buffers which are then synced to disk before
the write() system call returns. You are correct that the additional delay or
slowness of COOKED devices (as well as most of the delay using COOKED files) is
caused by this extra buffer-to-buffer data transfer and the additional overhead
the OS incurs managing that buffer cache.

However, your distinction between between UNIX files and lv and your equation of
lv to RAW device is inaccurate. Logical volume managers create BOTH a RAW and
a COOKED (block) device for the lv's (logical volumes) they manage. Filesystems
are built on top of a block special file representing one of these lv's. The
character device is a true RAW device and writes to that device are transferred
directly from the IDS buffers to the disk controller. Writes to the block or
COOKED device are made through the OS buffer cache and writes to any UNIX files
created on filesystems which have been built over these lv's COOKED device files
are made through the UNIX filesystem driver which ultimately hands the data to
the COOKED device driver adding another layer of overhead.

I dunno, maybe we're saying the same things and missing each other in the dark
here? Informix does NOT assume that it has direct access to the disk when it
writes to ordinary files or to COOKED devices, it checks the file type before
opening the chunk path. If it determines that the chunk is NOT a character
special device then it opens the chunk in O_SYNC mode for data security. IDS
does not need to do that for RAW devices.

Art S. Kagel
> Richard Thompson
> Data Manager
>
> Hertfordshire Health Informatics Service (HHIS)
> Charter House
> Parkway
> Welwyn Garden City
> Herts
> AL8 6JL
> 01707 390855 x2505
>
> -----Original Message-----
> From: ART KAGEL, .... [mailto:KAGEL@bloomberg.net]
> Sent: 26 August 2003 13:45
> To: ids@iiug.org
> Subject: Re: Is Logical Volume a raw device on HPUX? [1754]
>
> ----- Original Message -----
> From: R.... Kerhalkar <rohit.kerhalkar@hp.com>
> At: 8/26 8:23
>
> > Hi All,
> > We have Informix IDS 9.4 running on HP-UX11i with 4 disk each with 36GB
> Disk
> > Space mirrored in RAID 0+1 configuration. We would like to partition the
> > disk into various Logical Volumes to create chunks.
> Constructed from a stripe of 4 mirrored pairs I hope and not from mirroring
> two
> 4 drive stripe sets.
>
> > a. From Informix point of view, will the Logical Volumes be still called
> > 'Raw Device' or 'Cooked Data Files'?
> Each logical volume will have a 'Cooked' block device and a corresponding
> 'Raw'
> character device file. You want to use the RAW character device for your
> chunks. Remember to use symbolic links to the device files as the actual
> chunk
> paths and not the device paths themselves.
>
> > b. Is there any advantage in using Logical Volumes as opposed to cooked
> > files on HP-UX?
> Yes. There are three general classes of chunks - RAW devices, COOKED
> devices,
> and filesystem files. Due to the overhead of copying the IDS buffers
> to/from
> the COOKED device driver's cache before transferring between the device and
> IDS
> RAW devices are 15-25% faster than COOKED devices. Using a filesystem file
> adds
> the additional overheads of FS maintenance and the very real possibility
> that
> the chunk's sectors are not physically contiguous on disk making filesystem
> chunks an additional 5-15% slower than COOKED devices or overall 20-40%
> slower
> than RAW devices. This is what my testing has always shown. The so called
> 'fast' filesystems just move their performance to the lower range of the
> difference.
>
> So the short answer is 'Yes' RAW partitions are better than COOKED paritions
> or
> filesystem files on any platform.
>
> Art S. Kagel
>
> > Regards,
> > Rohit
> >
> >
> >



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