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Chapter 20. Solid-State Disk Deployment Guidelines
120
In addition, keep in mind that logical volumes, device-mapper targets, and md targets do not support
TRIM. As such, the default Red Hat Enterprise Linux 6 installation will not allow the use of the TRIM
command, since this install uses DM-linear targets.
Red Hat also warns that software RAID levels 1, 4, 5, and 6 are not recommended for use on SSDs.
During the initialization stage of these RAID levels, some RAID management utilities (such as mdadm)
write to all of the blocks on the storage device to ensure that checksums operate properly. This will
cause the performance of the SSD to degrade quickly.
At present, ext4 is the only fully-supported file system that supports TRIM. To enable TRIM commands
on a device, use the mount option discard. For example, to mount /dev/sda2 to /mnt with TRIM
enabled, run:
mount -t ext4 -o discard /dev/sda2 /mnt
By default, ext4 does not issue the TRIM command. This is mostly to avoid problems on devices which
may not properly implement the TRIM command. The Linux swap code will issue TRIM commands to
TRIM-enabled devices, and there is no option to control this behaviour.
20.2. Tuning Considerations
This section describes several factors to consider when configuring settings that may affect SSD
performance.
I/O Scheduler
Any I/O scheduler should perform well with most SSDs. However, as with any other storage type, Red
Hat recommends benchmarking to determine the optimal configuration for a given workload.
When using SSDs, Red Hat advises changing the I/O scheduler only for benchmarking particular
workloads. For more information about the different types of I/O schedulers, refer to the I/O Tuning
Guide (also provided by Red Hat). The following kernel document also contains instructions on how to
switch between I/O schedulers:
/usr/share/doc/kernel-version/Documentation/block/switching-sched.txt
Virtual Memory
Like the I/O scheduler, virtual memory (VM) subsystem requires no special tuning. Given the fast
nature of I/O on SSD, it should be possible to turn down the vm_dirty_background_ratio and
vm_dirty_ratio settings, as increased write-out activity should not negatively impact the latency
of other operations on the disk. However, this can generate more overall I/O and so is not generally
recommended without workload-specific testing.
Swap
An SSD can also be used as a swap device, and is likely to produce good page-out/page-in
performance.
- Red Hat Enterprise Linux 6 1
- Administration Guide 1
- Storage Administration Guide 2
- Enterprise Linux 6 3
- Edition 0 3
- 1. Document Conventions 9
- 1.2. Pull-quote Conventions 10
- 1.3. Notes and Warnings 11
- 2.1. Do You Need Help? 11
- 2.2. We Need Feedback! 12
- Overview 13
- Storage Considerations During 15
- Installation 15
- 2.3. Special Considerations 16
- LVM (Logical Volume Manager) 19
- 3.1. What is LVM2? 20
- 3.2. Using system-config-lvm 20
- 3.2.3. Migrating Extents 25
- 3.3. References 33
- Partitions 35
- 4.2. Creating a Partition 37
- 4.2.3. Add to /etc/fstab 38
- 4.3. Removing a Partition 39
- 4.4. Resizing a Partition 39
- File System Structure 41
- 5.2.1.2. The /boot/ Directory 43
- 5.2.1.3. The /dev/ Directory 43
- 5.2.1.9. The /proc/ Directory 45
- 5.2.1.11. The /srv/ Directory 45
- 5.2.1.12. The /sys/ Directory 45
- 5.2.1.13. The /usr/ Directory 46
- 5.2.1.14. The /var/ Directory 47
- The Ext3 File System 51
- New Mount Option: data_err 52
- More Efficient Storage Use 52
- The Ext4 File System 55
- Global File System 2 59
- The XFS File System 61
- 9.3. XFS Quota Management 63
- Setting Project Limits 64
- end: ino 1 offset 0 67
- mfile end: ino 1 offset 0 67
- Network File System (NFS) 69
- 10.1.1. Required Services 70
- 10.3. autofs 72
- 10.3.2. autofs Configuration 74
- 10.6.2. The exportfs Command 82
- 10.7. Securing NFS 84
- 10.7.2. Host Access in NFSv4 85
- 10.7.3. File Permissions 85
- 10.8. NFS and rpcbind 86
- 10.9. Using NFS over TCP 87
- 10.10. References 88
- FS-Cache 89
- 11.1. Performance Guarantee 90
- 11.2. Setting Up a Cache 90
- 11.3.1. Cache Sharing 91
- 11.5. Statistical Information 93
- 11.6. References 94
- Encrypted File System 95
- 12.2. Additional Information 96
- 13.1. What is RAID? 97
- 13.2. Who Should Use RAID? 97
- 13.3. RAID Types 97
- Software RAID 98
- 13.5. Linux RAID Subsystems 100
- 13.7. Configuring RAID Sets 101
- Swap Space 103
- 14.2.3. Creating a Swap File 104
- 14.3. Removing Swap Space 105
- 14.4. Moving Swap Space 106
- Disk Quotas 107
- 15.2. Managing Disk Quotas 110
- Best method for most systems 111
- 15.3. References 112
- Access Control Lists 113
- 16.3. Setting Default ACLs 114
- 16.4. Retrieving ACLs 114
- 16.7. References 116
- Write Barriers 117
- Disabling Write Caches 118
- Battery-Backed Write Caches 118
- High-End Arrays 119
- 18.2. Userspace Access 122
- 18.3. Standards 122
- 18.4. Stacking I/O Parameters 123
- 0, and minimum_io_size is a 124
- File System tools 125
- Solid-State Disk Deployment 131
- Guidelines 131
- 20.2. Tuning Considerations 132
- Online Storage Management 133
- 21.2. iSCSI 135
- 21.3. Persistent Naming 135
- 21.3.1. WWID 136
- Important 137
- local timeout=20 142
- local done=1 142
- 21.16.1. Fibre Channel 154
- SCSI Error Handler 156
- 21.16.3. iSCSI Root 157
- 21.18. Troubleshooting 158
- 22.1. Virtual Storage 161
- 22.2. DM-Multipath 161
- Appendix A. Revision History 163
- Glossary 165
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