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Old 08-02-2006, 05:05 PM   #1
Denes
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Registered: Mar 2004
Distribution: CentOS 4.3/4.5
Posts: 72

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Flash write on ext3 FS stalling


I am bulding a product where I need to do fairly fast reads/writes (under 100 milliseconds for 512 bytes) to a external Secure Digital flash reader running with fairly fast Secure Digital media (133x = ~20 mB/s). All reads/writes are done from/to a single file. Right now I am using CentOS4.3 for the distro. I am using a ext3 file system with data journaling enabled and committed every second. I do not use synchronous writes. Every once in a while, my program hangs in the fstream write (I only write 512 bytes at a time by the way) for over 100 milliseconds. It happens more often if I use fstream.flush. Is there anything I can do to prevent this from happening? Does anyone here know how to use the sys file system to minimize write latency or tweak other performace characteristics? I can't seem to find any specific documentation on this. I did find something on elvtune but this only works with 2.4 kernels.

BTW, I tried this with a ext3 file system in a file on top of vfat and it never stalls. But then again I have to manually do a sync() to make sure all the data gets there which takes 300 milliseconds. Also ReiserFS or XFS are not an option since I need as many distributions as possible to be able to read/write to the media and lots of them don't have this in the kernel by default.

Thanks!

Update: I changed the mount to flags to synchronous. I still get writes above 100ms. Interestingly when setting the file pointer using fstream.seekg even for reads I have timeout problems over 100ms occasionally. Any ideas why?

Last edited by Denes; 08-08-2006 at 03:35 PM.
 
Old 08-28-2006, 12:23 PM   #2
Denes
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Registered: Mar 2004
Distribution: CentOS 4.3/4.5
Posts: 72

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After doing more research I have found the problems were due to media. If I use higher quality media, the number of accesses above 100ms is greatly reduced. It turns out that in terms of SD if you want high quality with no errors you really must go high grade.
 
  


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