艾尔战记礼包怎么领:[Nemeth10] 8.8. Logical volume management

8.8. Logical volume management

Imaginea world in which you don’t know exactly how large a partition needs tobe. Six months after creating the partition, you discover that it ismuch too large, but that a neighboring partition doesn’t have enoughspace... Sound familiar? A logical volume manager lets you reallocatespace dynamically from the greedy partition to the needy partition.

Logical volume managementis essentially a supercharged and abstracted version of diskpartitioning. It groups individual storage devices into “volume groups.”The blocks in a volume group can then be allocated to “logicalvolumes,” which are represented by block device files and act like diskpartitions.

However, logical volumesare more flexible and powerful than disk partitions. Here are some ofthe magical operations a volume manager lets you carry out:

• Move logical volumes among different physical devices

• Grow and shrink logical volumes on the fly

• Take copy-on-write “snapshots” of logical volumes

• Replace on-line drives without interrupting service

• Incorporate mirroring or striping in your logical volumes

The components of a logicalvolume can be put together in various ways. Concatenation keeps eachdevice’s physical blocks together and lines the devices up one afteranother. Striping interleaves the components so that adjacent virtualblocks are actually spread over multiple physical disks. By reducingsingle-disk bottle-necks, striping can often provide higher bandwidthand lower latency.

LVM implementations

All our example systemssupport logical volume management, and with the exception of Solaris’sZFS, the systems are all quite similar.

In addition to ZFS, Solaris supports a previous generation of LVMcalled the Solaris Volume Manager, formerly Solstice DiskSuite. Thisvolume manager is still supported, but new deployments should use ZFS.

Linux’s volume manager, calledLVM2, is essentially a clone of HP-UX’s volume manager, which is itselfbased on software by Veritas. The commands for the two systems areessentially identical, but we show examples for both systems becausetheir ancillary commands are somewhat different. AIX’s system hassimilar abstractions but different command syntax. Table 8.4 illustrates the parallels among these three systems.

Table 8.4. Comparison of LVM commands

Inspect

Modify

pvdisplay

pvchange

pvdisplay

pvchange

lspv

chpv

Modify

Extend

Inspect

Check

vgchange

vgextend

vgdisplay

vgck

vgchange

vgextend

vgdisplay

–

chvg

extendvg

lsvg

–

Modify

Resize

lvchange

lvresize

lvchange

lvextend, lvreduce

chlv

extendlv

In addition to commands that deal with volume groups and logical volumes, Table 8.4 also shows a couple of commands that relate to “physical volumes.” A physical volume is a storage device that has had an LVM label applied; applying such a label is the first step to using the device through the LVM. Linux and HP-UX use pvcreate to apply a label, but AIX’s mkvg does it automatically. In addition to bookkeeping information, the label includes a unique ID to identify the device.

“Physicalvolume” is a somewhat misleading term because physical volumes need nothave a direct correspondence to physical devices. They can be disks, but they can also be disk partitions or RAID arrays. The LVM doesn’t care.

Linux logical volume management

You can control Linux’s LVM implementation (LVM2) with either a large group of simple commands (the ones illustrated in Table 8.4) or with the single lvmcommand and its various subcommands. These options are for all intentsand purposes identical; in fact, the individual commands are really justlinks to lvm, which looks to see how it’s been called to know how to behave. man lvm is a good introduction to the system and its tools.

A Linux LVM configuration proceeds in a few distinct phases:

• Creating (defining, really) and initializing physical volumes
  
  
• Adding the physical volumes to a volume group
  
  
• Creating logical volumes on the volume group
  
  

LVM commands start with letters that make it clear at which level of abstraction they operate: pv commands manipulate physical volumes, vg commands manipulate volume groups, and lv commands manipulate logical volumes. A few commands with the prefix lvm (e.g., lvmchange) operate on the system as a whole.

In the following example, we set up the /dev/md0 RAID 5 device we created on page 243 for use with LVMand create a logical volume. Since striping and redundancy have alreadybeen addressed by the underlying RAID configuration, we won’t make useof the corresponding LVM2 features, although they exist.

$ sudo pvcreate /dev/md0
Physical volume "/dev/md0" successfully created

Our physical device is now ready to be added to a volume group:

$ sudo vgcreate DEMO /dev/md0
Volume group "DEMO" successfully created

Although we’re using only asingle physical device in this example, we could of course addadditional devices. In this case, it would be strange to add anythingbut another RAID 5 array since there is no benefit to partialredundancy. DEMO is an arbitrary name that we’ve selected.

To step back and examine our handiwork, we use the vgdisplay command:

$ sudo vgdisplay DEMO
--- Volume group ---
VG Name                DEMO
System ID
Format                 lvm2
Metadata Areas         1
Metadata Sequence No   1
VG Access              read/write
VG Status              resizable
MAX LV                 0
Cur LV                 0
Open LV                0
Max PV                 0
Cur PV                 1
Act PV                 1
VG Size                975.99 GB
PE Size                4.00 MB
Total PE               249854
Alloc PE / Size        0 / 0
Free                   PE / Size249854 / 975.99 GB
VG UUID                NtbRLu-RqiQ-3Urt-iQZn-vEvJ-u0Th-FVYKWF

A “PE” is a physical extent, the allocation unit according to which the volume group is subdivided.

The final steps are tocreate the logical volume within DEMO and then to create a filesystemwithin that volume. We make the logical volume 100GB in size:

$ sudo lvcreate -L 100G -n web1 DEMO
Logical volume "web1" created

Most of LVM2’sinteresting options live at the logical volume level. That’s wherestriping, mirroring, and contiguous allocation would be requested if wewere using those features.

We can now access the volume through the device /dev/DEMO/web1. We discuss filesystems in general starting on page 254, but here is a quick overview of creating a standard filesystem so that we can demonstrate a few additional LVM tricks.

$ sudo mkfs /dev/DEMO/web1
...
$ sudo mkdir /mnt/web1
$ sudo mount /dev/DEMO/web1 /mnt/web1

Volume snapshots

Youcan create copy-on-write duplicates of any LVM2 logical volume, whetheror not it contains a filesystem. This feature is handy for creating aquiescent image of a filesystem to be backed up on tape, but unlike ZFSsnapshots, LVM2 snapshots are unfortunately not very useful as a generalmethod of version control.

The problem is that logicalvolumes are of fixed size. When you create one, storage space isallocated for it up front from the volume group. A copy-on-writeduplicate initially consumes no space, but as blocks are modified, thevolume manager must find space in which to store both the old and newversions. This space for modified blocks must be set aside when youcreate the snapshot, and like any LVM volume, the allocated storage is of fixed size.

Note that it does not matterwhether you modify the original volume or the snapshot (which bydefault is writable). Either way, the cost of duplicating the blocks ischarged to the snapshot. Snapshots’ allocations can be pared away byactivity on the source volume even when the snapshots themselves areidle.

If you do not allocate asmuch space for a snapshot as is consumed by the volume of which it is animage, you can potentially run out of space in the snapshot. That’smore catastrophic than it sounds because the volume manager then has noway to maintain a coherent image of the snapshot; additional storagespace is required just to keep the snapshot the same. The result of running out of space is that LVM stops maintaining the snapshot, and the snapshot becomes irrevocably corrupt.

So, as a matter of practice, LVM snapshots should be either short-lived or as large as their source volumes. So much for “lots of cheap virtual copies.”

To create /dev/DEMO/web1-snap as a snapshot of /dev/DEMO/web1, we would use the following command:

$ sudo lvcreate -L 100G -s -n web1-snap DEMO/web1

Note that the snapshot has its own name and that the source of the snapshot must be specified as volume_group/volume.

In theory, /mnt/web1should really be unmounted first to ensure the consistency of thefilesystem. In practice, ext4 will protect us against filesystemcorruption, although we may lose a few of the most recent data blockupdates. This is a perfectly reasonable compromise for a snapshot usedas a backup source.

To check on the status of your snapshots, run lvdisplay. If lvdisplaytells you that a snapshot is “inactive,” that means it has run out ofspace and should be deleted. There’s very little you can do with asnapshot once it reaches this point.

Resizing filesystems

Filesystemoverflows are more common than disk crashes, and one advantage oflogical volumes is that they’re much easier to juggle and resize thanare hard partitions. We have experienced everything from servers usedfor personal MP3 storage to a department full of email pack rats.

The logical volume managerdoesn’t know anything about the contents of its volumes, so you must doyour resizing at both the volume and filesystem levels. The orderdepends on the specific operation. Reductions must be filesystem-first,and enlargements must be volume-first. Don’t memorize these rules: justthink about what’s actually happening and use common sense.

Suppose that in our example, /mnt/web1has grown more than we predicted and needs another 10GB of space. Wefirst check the volume group to be sure additional space is available.

VG Name
System ID
Format
Metadata Areas
Metadata Sequence No
VG Access
VG Status
MAX LV
Cur LV
Open LV
Max PV
Cur PV
Act PV
VG Size
PE Size
Total PE
Alloc PE / Size

DEMO

lvm2
1
18
read/write
resizable
0
2
1
0
1
1
975.99 GB
4.00 MB
249854
51200 / 200.00 GB

Plenty of space is available, so we unmount the filesystem and use lvresize to add space to the logical volume.

$ sudo umount /mnt/web1
$ sudo lvchange -an DEMO/web1
$ sudo lvresize -L +10G DEMO/web1
$ sudo lvchange -ay DEMO/web1
Extending logical volume web1 to 110.00 GB
Logical volume web1 successfully resized

The lvchangecommands are needed to deactivate the volume for resizing and toreactivate it afterwards. This part is only needed because there is anexisting snapshot of web1 from our previous example. After the resizeoperation, the snapshot will“see” the additional 10GB of allocated space, but since the filesystemit contains is only 100GB in size, the snapshot will still be usable.

We can now resize the filesytem with resize2fs. (The 2 comes from the original ext2 filesystem, but the command supports all versions of ext.) Since resize2fscan determine the size of the new filesystem from the volume, we don’tneed to specify the new size explicitly. We would have to do so whenshrinking the filesystem.

$ sudo resize2fs /dev/DEMO/web1
resize2fs 1.41.9 (22-Aug-2009)
Please run 'e2fsck -f /dev/DEMO/web1' first.

Oops! resize2fs forces you to double-check the consistency of the filesystem before resizing.

$ sudo e2fsck -f /dev/DEMO/web1
e2fsck 1.41.9 (22-Aug-2009)
Pass 1: Checking inodes, blocks, and sizes
...
/dev/DEMO/web1: 6432/6553600 files (0.1% non-contiguous), 473045/26214400
     blocks
$ sudo resize2fs /dev/DEMO/web1
resize2fs 1.41.9 (22-Aug-2009)
Resizing the filesystem on /dev/DEMO/web1 to 28835840 (4k) blocks.
The filesystem on /dev/DEMO/web1 is now 28835840 blocks long.

That’s it! Examining the output of df again shows the changes:

$ sudo mount /dev/DEMO/web1 /mnt/web1
$ df -h /mnt/web1
Filesystem             Size   Used  Avail  Use%  Mounted on
/dev/mapper/DEMO-web  1109G   188M  103G     1%   /mnt/web1

HP-UX logical volume management

As of HP-UX 10.20, HPprovides a full logical volume manager. It’s a nice addition, especiallywhen you consider that HP-UX formerly did not even support the notionof disk partitions. The volume manager is called LVM, just as on Linux, although the HP-UX version is in fact the original. (Really, it’s Veritas software...)

As a simple example of LVMwrangling, here’s how you would configure a 75GB hard disk for use withthe logical volume manager. If you have read through the Linux exampleabove, the following procedure will seem eerily familiar. There are afew minor differences, but the overall process is essentially the same.

The pvcreate command identifies physical volumes.

$ sudo pvcreate /dev/rdisk/disk4
Creating "/etc/lvmtab_p".
Physical volume "/dev/rdisk/disk4" has been successfully created.

If you will be using the disk as a boot disk, add the -B option to pvcreate to reserve space for a boot block, then run mkboot to install it.

After defining the disk as a physical volume, you add it to a new volume group with the vgcreate command. Two metadata formats exist for volume groups, versions 1.0 and 2.0. You specify which version you want with the -Voption when creating a volume group; version 1.0 remains the default.Version 2.0 has higher size limits, but it’s not usable for boot devicesor swap volumes. Even version 1.0 metadata has quite generous limits,so it should be fine for most uses. You can see the exact limits with lvmadm. For reference, here are the limits for 1.0:

$ sudo lvmadm -t -V 1.0
--- LVM Limits ---
VG Version                1.0
Max VG Size (Tbytes)      510
Max LV Size (Tbytes)      16
Max PV Size (Tbytes)      2
Max VGs                   256
Max LVs                   255
Max PVs                   255
Max Mirrors               2
Max Stripes               255
Max Stripe Size (Kbytes)  32768
Max LXs per LV            65535
Max PXs per PV            65535
Max Extent Size (Mbytes)  256

You can add extra disks to a volume group with vgextend, but this example volume group contains only a single disk.

$ sudo vgcreate vg01 /dev/disk/disk4
Increased the number of physical extents per physical volume to 17501.
Volume group "/dev/vg01" has been successfully created.
Volume Group configuration for /dev/vg01 has been saved in
    /etc/lvmconf/vg01.con

Once your disks have been added toa convenient volume group, you can split the volume group’s pool ofdisk space back into logical volumes. The lvcreate command creates a new logical volume. Specify the size of the volume in megabytes with the -L flag or in logical extents (typically 4MiB) with the -l flag. Sizes specified in MiB are rounded up to the nearest multiple of the logical extent size.

To assess the amount of free space remaining in a volume group, run vgdisplay vgname as root. The output includes the extent size and the number of unallocated extents.

$ sudo lvcreate -L 25000 -n web1 vg01
Logical volume "/dev/vg01/web1" has been successfully created with character
     device "/dev/vg01/rweb1".
Logical volume "/dev/vg01/web1" has been successfully extended.
Volume Group configuration for /dev/vg01 has been saved in
    /etc/lvmconf/vg01.conf

  

Thecommand above creates a 25GB logical volume named web1. Once you’vecreated your logical volumes, you can verify them by running vgdisplay -v /dev/vgname to double-check their sizes and make sure they were set up correctly.

In most scenarios, you would then go on to create a filesystem on /dev/vg01/web1 and arrange for it to be mounted at boot time. See page 258 for details.

Another common way to create a logical volume is to use lvcreate to create a zero-length volume and then use lvextendto add storage to it. That way, you can specify exactly which physicalvolumes in the volume group should compose the logical volume. If youallocate space with lvcreate(as we did above), it simply uses free extents from any availablephysical volumes in the volume group—good enough for most situations.

As in Linux, striping (which HP-UX’s LVMrefers to as “distributed allocation”) and mirroring are features atthe logical volume level. You can request them at the time the logicalvolume is created with lvcreate, or later with lvchange.In contrast to Linux, the logical volume manager does not allowsnapshots. However, temporary snapshots are available as a feature ofHP’s VxFS filesystem.

If you plan to use a logicalvolume as a boot or swap device or to store system core dumps, you mustspecify contiguous allocation and turn off bad block remapping with the -C and -r flags to lvcreate, as shown below.^[16]

^[16]HP-UX limitations require swap space to reside in the first 2GiB of thephysical disk and the boot volume to be the first logical volume. The1.5GB root and 500MB swap shown here were chosen to work around theseconstraints. You can have a root partition that is larger than thesevalues, but you must then have separate boot and root volumes. See theman page for lvlnboot for more details.

# lvcreate -C y -r n -L 1500 -n root vg01
Logical volume "/dev/vg01/root" has been successfully created with character
     device "/dev/vg01/rroot".
Logical volume "/dev/vg01/root" has been successfully extended.
Volume Group configuration for /dev/vg01 has been saved in
    /etc/lvmconf/vg01.conf
# lvcreate -C y -r n -L 500 -n swap vg01
Logical volume "/dev/vg01/swap" has been successfully created with character
     device "/dev/vg01/rswap".
...

  

You must then run the lvlnboot command to notify the system of the new root and swap volumes. See the man page for lvlnboot for more information about the special procedures for creating boot, swap, and dump volumes.

AIX logical volume management

AIX’s logical volumemanager uses a different command set from the volume managers of Linuxand HP-UX, but its underlying architecture and approach are similar. Onepotentially confusing point is that AIX calls the objects more commonlyknown as extents (that is, the units of space allocation within avolume group) “partitions.” Because the entities normally referred to aspartitions do not existin AIX, there is no ambiguity within the AIX sphere itself. However,tourists visiting from other systems may wish to bring along an AIXphrase book.

In other respects—physicalvolume, volume group, logical volume—AIX terminology is standard. TheSMIT interface for logical volume management is pretty complete, but youcan also use the commands listed in Table 8.4.

The following fourcommands create a volume group called webvg, a logical volume calledweb1 within it, and a JFS2 filesystem inside web1. The filesystem isthen mounted in /mnt/web1.

$ sudo mkvg -y webvg hdisk1
webvg
$ sudo crfs -v jfs2 -g webvg -m /mnt/web1 -a size=25G
File system created successfully.
26213396 kilobytes total disk space.
New File System size is 52428800
$ sudo mkdir /mnt/web1
$ sudo mount /mnt/web1

AIX does not require you to label disks to turn them into physical volumes. mkvg and extendvg automatically label disks as part of the induction process. Note that mkvg takes a device name and not the path to a disk device.

You can create the logical volume and the filesystem inside it in separate steps (with mklv and mkfs, respectively), but crfs performs both tasks for you and updates /etc/filesystems as well. The exact name of the logical volume device that holds the filesystem is made up for you in the crfs scenario, but you can determine it by inspecting /etc/filesystems or running mount. (On the other hand, it can be hard to unscramble filesystems in the event of problems if the volumes all have generic names.)

If you run mklvdirectly, you can specify not only a device name of your choosing butalso various options to the volume manager such as striping andmirroring configurations. Snapshots are implemented through the JFS2filesystem and not through the volume manager.