[lustre-discuss] Small sequential reads on cached file are slow

[Hans Henrik Happe]

Hi,

While testing small sequential reads I noticed that Lustre is more than 
10 times slower than local fs when reading cached data. So basically no 
network I/O to OSSes. I wanted to check if readahead was working for 
this small I/O case, but it seems that cached case isn't.

The client is running Lustre 2.8 and CentOS 6.8.

Lustre:

$ dd if=file of=/dev/null bs=512
2097152+0 records in
2097152+0 records out
1073741824 bytes (1.1 GB) copied, 20.5081 s, 52.4 MB/s

$ dd if=file of=/dev/null bs=4k
262144+0 records in
262144+0 records out
1073741824 bytes (1.1 GB) copied, 2.91177 s, 369 MB/s

$ dd if=file of=/dev/null bs=1M
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 0.160732 s, 6.7 GB/s

Local fs:

$ dd if=/tmp/file of=/dev/null bs=512
2097152+0 records in
2097152+0 records out
1073741824 bytes (1.1 GB) copied, 1.56432 s, 686 MB/s

$ dd if=/tmp/file of=/dev/null bs=4k
262144+0 records in
262144+0 records out
1073741824 bytes (1.1 GB) copied, 0.275451 s, 3.9 GB/s

$ dd if=/tmp/file of=/dev/null bs=1M
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 0.148798 s, 7.2 GB/s


Is this a known issue?

Cheers,
Hans Henrik Happe