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Wednesday, November 4, 2009

Potential problems with "Write Back Caching"

For the modern hd drives there is something what can make some problems when the operating system crushed. It is also got to know for troubleshooting and tuning of os and application like data base for example.

Try it for your self:

# man hdparm
# man sdparm

To disable write cache execute:
#hdparm -W0 /dev/hdX | device
#sdparm --set WCE=0

To enable write cache execute:
#hdparm -W1 device
#sdparm --set WCE=1

# sdparm -a -e /dev/sda
# sdparm --page=ca --long /dev/sda
/dev/sda: ATA Hitachi HTS72201 DCDO
Direct access device specific parameters: WP=0 DPOFUA=0
Caching (SBC) [ca] mode page:
IC 0 Initiator control
ABPF 0 Abort pre-fetch
CAP 0 Caching analysis permitted
DISC 0 Discontinuity
SIZE 0 Size enable
WCE 1 Write cache enable
MF 0 Multiplication factor
RCD 0 Read cache disable
DRRP 0 Demand read retention priority
WRP 0 Write retention priority
DPTL 0 Disable pre-fetch transfer length
MIPF 0 Minimum pre-fetch
MAPF 0 Maximum pre-fetch
MAPFC 0 Maximum pre-fetch ceiling
FSW 0 Force sequential write
LBCSS 0 Logical block cache segment size
DRA 0 Disable read ahead
NV_DIS 0 Non-volatile cache disable
NCS 0 Number of cache segments
CSS 0 Cache segment size

a few links in net:
The sdparm utility
Praxis-Know-how: RAID-Controller optimal konfigurieren
MySql InnoDB Configuration
Experiments on Disk Write Back Caches in Linux
The Linux Page Cache and pdflush: Theory of Operation and Tuning for Write-Heavy Loads
Linux IDE Problem beim Write Back Caching
turning on dma and write back cache on SATA

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