Messages in this thread | | | Subject | Re: read on /dev/hd? ? | Date | Sun, 23 Aug 1998 21:06:25 -0400 (EDT) | From | "Vladimir Dergachev" <> |
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> > > Vladimir Dergachev wrote: > > If I make a read on /dev/hd? as a file (after doing lseek) > > would the data I read put in the buffer and later reused > > if filesystem tried to access the disk ? > > Do you mean "will be reads go through the buffer cache"? The answer is yes.
The question is this : suppose that /dev/hda4 has an ext2 partition on it. Suppose also that a file in this partition has blocks 1,2,1024. Suppose also that a program read /dev/hda4 directly (as a file (and it doesn't mmap)) so that blocks 1 2 and 1024 were read. Now if I were to read the same file but using the files inode (i.e. thru the ext2 partition) would the VFS recognize that I am going for the same blocks ? (I think that the answer to this is yes though I am not sure). The other part of the question is that if the file was executable and I tried to execute it (or mmap) will the same happen ? Apparently there is big difference to the kernel whether the file is mmaped or just read.. So far I couldn't make my test program load /dev/hda4 in such a way so that the blocks get reused when the files are read thru ext2.. But it could be well that I got the block numbers wrong.. (Btw can anybody tell me if I talk about block 731 should I lseek to 512*731 or 1024*731 ? I think I heard somewhere that SCSI uses 512byte blocks but I actually have IDE drives so I don't know)
> > > Is it dangerous to read a mounted device ? (only read) > > Depends on how you define "dangerous". You can't hurt the filesystem by > just reading. However if the filesystem is active, the application doing > the reads might get nasty surprises as things change under it. For > best safety, remount the filesystem read-only also. >
Thanks..
> -Mitch >
Vladimir Dergachev
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