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Re: LBA to File?

To: Bogdan Costescu <bogdan.costescu@xxxxxxxxxxxxxxxxxxxxx>
Subject: Re: LBA to File?
From: Michael Sinz <msinz@xxxxxxxxx>
Date: Fri, 07 Mar 2003 09:15:57 -0500
Cc: Eric Sandeen <sandeen@xxxxxxx>, Andi Kleen <ak@xxxxxxx>, "Buzbee, James" <James.Buzbee@xxxxxxxxxxxx>, linux-xfs@xxxxxxxxxxx
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Bogdan Costescu wrote:
On 6 Mar 2003, Eric Sandeen wrote:


Hm, I suppose that for the truly paranoid, with cheap IDE disks,


No, not only for them. The frequent access exists for all drives and will destroy a high-end SCSI disk just as well, but maybe not as fast.

I would think that all of this "worry" is not a major issue.  One
of the reasons you may find more "remapped" blocks in the highly used
areas is that they drive had more chance to notice a questionable
sector.  (That is a sector where error correction was needed to
recover the data beyond the norm and thus it chose to relocate the
sector)

If you are worried that such usage patterns can be a problem, just look
at filesystems such as FAT/FAT32/VFAT/etc.  They always must write to
the front of the disk (fat table) and read from that same area (except
when cached)  There is no noticable higher failure rate of drives due
to FAT file system usage (don't count Windows re-install and file
corruption as disk failures :-)

NTFS also has the central allocation bitmap and control structures.

The main problem with the "low end" IDE drives is that they are very
fragile bits of mechanical equipment.  Very small head to disk spacing
and very high rotational rates and very high bit density means that
failures will happen due to mechanical reasons.  As you cut your costs
and try to pump out millions of these units, the MTBF will tend to
drop until you get some new technique to make them more reliable.
Then then push the limits and make the drives double their bit
density and half the head flying height and the cycle starts over.

Given the track record of XFS and other file systems and usage
patterns that cause certain areas of disks to be used much more
often than others, I have not seen a major failure mode in many
years.  (There was the lubrication failure mode from many
years ago, but that has long since been solved)

That is not to say that we should not look at the fact that there
are hot zones in where writing happens.  The fact that there are
hot zones (multiple) means that there is more seeking going on than
you might want (albeit it may be needed).  (And each seek does
cause some mechanical wear and poses potential mechanical failure)

--
Michael Sinz -- Director, Systems Engineering -- Worldgate Communications
A master's secrets are only as good as
        the master's ability to explain them to others.


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