On Tue, 12 Aug 2008, Ron Mayer wrote:
Really old software (notably 2.4 linux kernels) didn't send
cache synchronizing commands for SCSI nor either ATA; but
it seems well thought through in the 2.6 kernels as described
in the Linux kernel documentation.
http://www.mjmwired.net/kernel/Documentation/block/barrier.txt
If you've drank the kool-aid you might believe that. When I see people
asking about this in early 2008 at
http://thread.gmane.org/gmane.linux.kernel/646040 and serious disk driver
hacker Jeff Garzik says "It's completely ridiculous that we default to an
unsafe fsync." [ http://thread.gmane.org/gmane.linux.kernel/646040 ], I
don't know about you but that barrier documentation doesn't make me feel
warm and safe anymore.
If you do have a disk where you need to disable write caches,
I'd love to know the name of the disk and see the output of
of "hdparm -I /dev/sd***" to see if it claims to support such
cache flushes.
The below disk writes impossibly fast when I issue a sequence of fsync
writes to it under the CentOS 5 Linux I was running on it. Should only be
possible to do at most 120/second since it's 7200 RPM, and if I poke it
with "hdparm -W0" first it behaves. The drive is a known piece of junk
from circa 2004, and it's worth noting that it's an ext3 filesystem in a
md0 RAID-1 array (aren't there issues with md and the barriers?)
# hdparm -I /dev/hde
/dev/hde:
ATA device, with non-removable media
Model Number: Maxtor 6Y250P0
Serial Number: Y62K95PE
Firmware Revision: YAR41BW0
Standards:
Used: ATA/ATAPI-7 T13 1532D revision 0
Supported: 7 6 5 4
Configuration:
Logical max current
cylinders 16383 65535
heads 16 1
sectors/track 63 63
--
CHS current addressable sectors: 4128705
LBA user addressable sectors: 268435455
LBA48 user addressable sectors: 490234752
device size with M = 1024*1024: 239372 MBytes
device size with M = 1000*1000: 251000 MBytes (251 GB)
Capabilities:
LBA, IORDY(can be disabled)
Standby timer values: spec'd by Standard, no device specific
minimum
R/W multiple sector transfer: Max = 16 Current = 16
Advanced power management level: unknown setting (0x0000)
Recommended acoustic management value: 192, current value: 254
DMA: mdma0 mdma1 mdma2 udma0 udma1 udma2 udma3 udma4 udma5 *udma6
Cycle time: min=120ns recommended=120ns
PIO: pio0 pio1 pio2 pio3 pio4
Cycle time: no flow control=120ns IORDY flow control=120ns
Commands/features:
Enabled Supported:
* SMART feature set
Security Mode feature set
* Power Management feature set
* Write cache
* Look-ahead
* Host Protected Area feature set
* WRITE_VERIFY command
* WRITE_BUFFER command
* READ_BUFFER command
* NOP cmd
* DOWNLOAD_MICROCODE
Advanced Power Management feature set
SET_MAX security extension
* Automatic Acoustic Management feature set
* 48-bit Address feature set
* Device Configuration Overlay feature set
* Mandatory FLUSH_CACHE
* FLUSH_CACHE_EXT
* SMART error logging
* SMART self-test
--
* Greg Smith gsmith@xxxxxxxxxxxxx http://www.gregsmith.com Baltimore, MD
