Re: Filesystem benchmarking for pg 8.3.3 server

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@gregsmith.com http://www.gregsmith.com Baltimore, MD