SSD & CentOS 6+
SSDs can
be divided into three classes, based on throughput:
- The first class
of SSDs use a PCI-Express connection, which offers the fastest I/O
throughput compared to other classes. This class also has a very low
latency for random access.
- The second class
uses the traditional SATA connection, and features fast random access for
read and write operations (though not as fast as SSDs that use PCI-Express
connection).
- The third class
also uses SATA, but the performance of SSDs in this class do not differ
substantially from devices that use 7200rpm rotational disks.
For all three
classes, performance degrades as the number of used blocks approaches the disk
capacity. The degree of performance impact varies greatly by vendor. However,
all devices experience some degradation.
Enabling
discard support is most useful when there is available
free space on the file system, but the file system has already written to most
logical blocks on the underlying storage device.
At present, ext4 is
the only fully-supported file system that supports
discard.
To enable
discard commands on a device, use the mount option discard. For example, to mount /dev/sda2 to /data with discard enabled, run:
mount -t
ext4 -o discard /dev/sdb1 /data
By default, ext4 does
not issue the
discard command.
This is mostly to
avoid problems on devices which may not properly implement the
discard command. The Linux swap code will issue discard commands to discard-enabled devices, and there is no option to
control this behavior.
There are basically two
big things to do
- enable
discard/trim support in the file system
- Limit write operations to the SSD. You want to
enable discard to deal with underlying drive specific performance degradation
that will happen over time
Move /tmp to ram.
This depends on how much ram you have and how much /tmp space you use, but it
really helps to limit writes to the SSD
Change the mount options
for your ext4 filesystem
-
change data
from the default of ordered to writeback. This reduces the journal data to
meta-data only, thus limiting writes.
-
change the
commit value from the default of 5 seconds to 10, or 15. This gets you a 2 to
3x write savings, though you risk losing a bit more data if you lose power.
-
If you don't
need access time records (mtime and ctime should be enough for most folks)
disabling atime will also limit write operations.
Ex: /dev/sdb1 /
ext4
defaults,discard,data=writeback,noatime,commit=15 1 1
A/ mount file systems
with the "noatime" option.
"noatime" prevents the file system from updating file access times.
"noatime" includes "nodiratime" (prevents directory access time updates).
"noatime" prevents the file system from updating file access times.
"noatime" includes "nodiratime" (prevents directory access time updates).
Consider whether you need
journals
File system
journals make file system reconstruction (fsck for
example) possible, and running without any journal makes catastrophic
file system failure far more likely. On the other hand, journals
require extra write operations, Sometimes it's easy enough to
reconstruct a file system that it's not worth the cost in terms of SSD
write operations.
example) possible, and running without any journal makes catastrophic
file system failure far more likely. On the other hand, journals
require extra write operations, Sometimes it's easy enough to
reconstruct a file system that it's not worth the cost in terms of SSD
write operations.
echo "noop"
> /sys/block/sda/queue/scheduler
in /etc/rc.d/rc.local.
in /etc/rc.d/rc.local.
OR
one can eliminate journaling altogether by doing this to a
unmounted or read-only ext4
tune2fs -O ^has_journal /dev/sda1
tune2fs -O ^has_journal /dev/sda1
Then there will be no journal. While you are at it the command:
tune2fs -r 1024 /dev/sda1
will reduce the root-reserved blocks to 1k (4MB)
tune2fs -r 1024 /dev/sda1
will reduce the root-reserved blocks to 1k (4MB)
Get rid of /tmp
Put it on ramdisk. The
X11 server makes a huge number of tiny
writes to /tmp and if these flush to disk at a high rate it creates a
load of writes. Adding this line to /etc/fstab does the trick.
writes to /tmp and if these flush to disk at a high rate it creates a
load of writes. Adding this line to /etc/fstab does the trick.
none /tmp tmpfs defaults
0 0
To enable trim, use the "discard' mount
option in fstab for all mounted partitions from the SSD. The "swap"
partition automatically uses TRIM. For example from /etc/fstab ....
Code:
UUID=ffc6a6fc-1bf3-4f25-ab17-067b0515e85a / ext4 noatime,discard,data=writeback 1 1
UUID=24941fe7-6d23-4969-951e-0872d4b81b0e /home ext4 noatime,discard,data=writeback 1 2
UUID=2bbebe51-c0f8-4296-ba0a-e715bf84dc67 swap swap defaults 0 0
Few useful Commands
fdisk
-u -l /dev/sdX
cat
/sys/block/sda/queue/scheduler
Use
iotop -oPa and sort by disk writes to see how much programs are writing to disk
Using “tune2fs –l /dev/sdb”, we can see the file system block
size is 4KB with journal turned on.
Using
“more /sys/fs/ext4/sdc/lifetime_write_kbytes” to see how much has been written
to the disk so far
stop all application/System services on the box,
unmount sdbX and sdcX
run tune2fs to set stride, stripe-width options
change mounting option on sdb, sdc to use noatime
mount again
mkfs.ext4 -b 1024 -E stride=128,stripe-width=128 -O ^has_journal /dev/sda1
Ref
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