Tag: FreeBSD

Here is a quick overview of the disks used in my tests:

Brand	Model	Capacity	Speed	Interface	Notes
Western Digital	WD4000AAKS	400 GB	7,200 rpm	SATA 3 Gbps
Western Digital	WD10EARS	1 TB	> 5,400 rpm	SATA 3 Gbps	1
Western Digital	WD20EARS	2 TB	> 5,400 rpm	SATA 3 Gbps	2
Western Digital	WD20EADS	2 TB	> 5,400 rpm	SATA 3 Gbps	3
Hitachi	HDS722020ALA330	2 TB	7,200 rpm	SATA 3 Gbps	3

The computer runs FreeBSD 9 on an Intel E6600 with an ICH9 chipset and 4 GB RAM. For convenience, the disks were tested in an Akasa Duo Dock connected by eSATA cable to one of the ICH9 SATA ports.

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”

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”