Benchmarking Advanced Format drives

Important: due to a bug in my benchmark program, the tps numbers in this post are incorrect. See here for the correct numbers.

In the previous post, I discussed Western Digital’s “Advanced Format” drives and the problems caused by their misreporting their real, physical sector size.

I wrote a benchmark utility to demonstrate the performance penalty of unaligned accesses and uncover a drive’s physical sector size. What it does is write blocks of zeroes varying size at regular intervals. For each block size, it writes a total of 128 MB at intervals of four times the block size, and at an offset that varies from 512 bytes up to half of the block size. Continue reading “Benchmarking Advanced Format drives”

Exploring WD Advanced Format drives

I’ve been playing with WD Green disks, trying to solve the 4,096-byte sector problem. To summarize, Western Digital have started to move from 512-byte sectors to 4,096-byte sectors in order to reduce overhead and thereby increase the amount of data that can be stored on the same amount of platters with the same density. These disks (specifically, the EARS and AARS series) emulate 512-byte sectors for compatibility with older BIOSes and operating systems, but the problem is that they report 512-byte logical and physical sectors instead of 512/4,096.

If the length of a write operation is not a multiple of 4,096, or it does not begin at an address divisible by 4,096, either the beginning or the end of the operation, or both, will cover only part of a sector. This requires the disk to do a read-modify-write operation, meaning that it has to read a complete 4,096-byte sector, update parts of it, and write it back. This is extremely inefficient, as I will demonstrate later. Continue reading “Exploring WD Advanced Format drives”

Old history

I am the current maintainer of OpenSSH for FreeBSD, and have been since 2002. I am also the author and maintainer of the PAM implementation used by FreeBSD, and of several of the accompanying PAM modules. Finally, I was a member of the FreeBSD Security Team for several years, served as Assistant Security Officer and Acting Security Officer, and authored or co-authored around 20 security advisories between 2002 and 2004.

I have been asked to comment on SecurityFocus advisories 7467 and 7482, regarding timing attacks against certain versions of OpenSSH that were distributed with FreeBSD 4.x and 5.x releases.

The short version is that no FreeBSD 4.x or 5.x release was ever vulnerable. Read on for the long version. Continue reading “Old history”