Linda Walsh <[EMAIL PROTECTED]> writes: > I started with 2 files about 250MB in length (random, compressed data) > and wanted to create a 256MB file. I thought to concatenate the > 256MB files with cat and use dd to read a fixed amount to an output > file: > cat testfile-250M testfile.gz | dd of=250MB bs=1M count=256 > > Is something "wrong" with the above"?...because it didn't work.
In a nutshell, use "head -c 256M" instead. dd reads data in units of the input block size. In the course it tries to read up to the input block size in one go (one read syscall). If it gets less than that it continues with the partial block, but counts it as if it was a full block. A pipe only guarantees $(ulimit -p) * 512 bytes (4Kb typically) of data atomically, anything bigger can be delivered in several chunks. Thus for a block size of 256MB the likelyhood of a partial block is rather high. Andreas. -- Andreas Schwab, SuSE Labs, [EMAIL PROTECTED] SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany PGP key fingerprint = 58CA 54C7 6D53 942B 1756 01D3 44D5 214B 8276 4ED5 "And now for something completely different." _______________________________________________ Bug-coreutils mailing list Bug-coreutils@gnu.org http://lists.gnu.org/mailman/listinfo/bug-coreutils
