Re: Large USB storage devices cause df to hang and cpu load reaches 100%

On Wed, Aug 01, 2007 at 01:14:40PM -0400, Alan Stern wrote:
> On Wed, 1 Aug 2007, Sergey Vlasov wrote:
> 
> > On Tue, Jul 31, 2007 at 11:08:37PM +0100, Kostas Peletidis wrote:
> > > Branden Sletteland wrote:
> > > > I have also noticed this delay and have through instrumenting code
> > > > have found that every block in the device gets read in.  I never
> > > > checked if it was something in the df, filesystem, SCSI, or USB
> > > > subsystems that was causing this.  I am usually doing df on USB
> > > > storage devices formated as FAT 32 w/ and w/o LBA.
> > > >
> > > > Branden
> > > >
> > > >   
> > > Thanks for your reply Branden. This explains why df would delay for a 
> > > whole minute to terminate normally with a 500GB usb disk attached and 
> > > only for a second or two with a 512MB usb stick attached. Btw both are vfat.
[...]
> > Without "usefree" the vfat code in 2.6.22 will read the whole FAT
> > to count free clusters on the filesystem.
> 
> But reading the whole FAT is very different from reading the entire 
> partition.

Reading the entire 500GB in one minute?  I would really like to have
such disk :)

Most likely "every block" here really means "a large sequential read".
Not sure what cluster size would be used for a 500GB FAT32 partition,
but with 4KB clusters the FAT size would be about 500MB - reading it
over USB could probably take about a minute.

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