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Full Discussion: SAN vs. Local disk.
Top Forums UNIX for Beginners Questions & Answers SAN vs. Local disk. Post 303031279 by bakunin on Monday 25th of February 2019 09:27:32 AM
Old 02-25-2019
Quote:
Originally Posted by ikx
I am in the market looking to purchase a new E950 server and I am trying to decide between using local SSD drives or SSD based SAN. The application that will be running on this server is read-intensive so I am looking for the most optimal configuration to support this application. There are no other servers or applications that will use this SAN (if I decide to go that route.) The deciding factor to me is Performance regardless of the hardware cost (granted I don't want to pay for something I end up not using.) SAN or local SSD (both running RAID 10)? This is really the question I am trying to answer before I pull the trigger and complete this purchase. Any insights from this community is greatly appreciated.
There are a few more points to consider IMHO:

1) disks (regardless which technology) will over time malfunction and need to be replaced. There is some effort involved in such a replacement. SAN systems have usually ways to - more or less - "effortless" replacing disks built in because they usually are used to deal with a lot of disks and the chance that one disk malfunctions rises with the number of disks involved. You might want to do some risk calculation based on how often on average you expect a disk to break (there is usually a "MTBF" - "meantime between failure" number available), how long you expect the change to take and how much a downtime of the expected duration will cost.

2) SANs and local disks do differ in the way they are attached to the system. Local disks may use SCSI or the "M.2" interface. Notice that you cannot have several local (SSD-) disks attached via M.2. SAN, onthe other hand, may use a FC-connection or even several FC-connections in parallel (FC-drivers allow this for redundancy as well as load-balancing). The capacity of such FC-connections may by far exceed the speed of local disks. On the other hand you will need not only a SAN but also a FC-switch (Brocade are the most wide-spread) and the administration ("zoning") will be more complicated.

3) SANs - if set up with this in mind - may add redundancy and thus high-availability to the system. Again, it depends on the system, its purpose, etc.. to calculate properly the risk of it failing for some amount of time. Calculate how much it would set you back to have the system offline for: 1 hour / 1day / 1 week and this will give you an idea how much money spent on preventing these kinds of desasters is worthwhile.

4) SAN systems for themselves are rather expensive. To alleviate this they become cheaper and cheaper (in comparison to local disks) the more you virtualise and the more systems use it. So, a plan to buy a SAN system for a single system only might be on the expensive side but with a expectation of adding other systems using it too the costs may still be reasonable. So you may want to rethink your immediate problem in a more gobal context.

5) Notice that for a system optimised for speed you need an adequate backup solution. For this you also need a disaster scenario plan to estimate what the costs could be and hence how much the prevention may cost. Then you know how fast a recovery needs to be and therefore which technologies you need to employ to get that speed. You can use a SAN also for snapshots (perhaps quickest way of recovering a "point in time"), very fast medium to put an online backup to, only then migrating it to slower media like tapes and similar solutions. You might want to take that also into consideration.

I hope this helps.

bakunin
 

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vxdarestore(1M) 														   vxdarestore(1M)

NAME
vxdarestore - restore simple or nopriv disk access records SYNOPSIS
/etc/vx/bin/vxdarestore DESCRIPTION
The vxdarestore utility is used to restore persistent simple or nopriv disk access (da) records that have failed due to changing the naming scheme used by vxconfigd from c#t#d#-based to enclosure-based. The use of vxdarestore is required if you use the vxdiskadm command to change from the c#t#d#-based to the enclosure-based naming scheme. As a result, some existing persistent simple or nopriv disks go into the "error" state and the VxVM objects on those disks fail. vxdarestore may be used to restore the disk access records that have failed. The utility also recovers the VxVM objects on the failed disk access records. Note: vxdarestore may only be run when vxconfigd is using the enclosure-based naming scheme. Note: You can use the command vxdisk list da_name to discover whether a disk access record is persistent. The record is non-persistent if the flags field includes the flag autoconfig; otherwise it is persistent. The following sections describe how to use the vxdarestore utility under various conditions. Persistent Simple/Nopriv Disks in the rootdg Disk Group If all persistent simple or nopriv disks in the rootdg disk group go into the "error" state, use the following procedure: 1. Use the vxdiskadm command to change back to the c#t#d# based naming scheme. 2. Either shut down and reboot the host, or run the following command: vxconfigd -kr reset 3. If you want to use the enclosure-based naming scheme, add a non-persistent simple disk to the rootdg disk group, use vxdiskadm to change to the enclosure-based naming scheme, and then run vxdarestore. Note: If not all the disks in rootdg go into the error state, simply running vxdarestore restores those disks in the error state and the objects that that they contain. Persistent Simple/Nopriv Disks in Disk Groups other than rootdg If all disk access records in an imported disk group consist only of persistent simple and/or nopriv disks, the disk group is put in the "online dgdisabled" state after changing to the enclosure-based naming scheme. For such disk groups, perform the following steps: 1. Deport the disk group using the following command: vxdg deport diskgroup 2. Run the vxdarestore command. 3. Re-import the disk group using the following command: vxdg import diskgroup NOTES
Use of the vxdarestore command is not required in the following cases: o If there are no persistent simple or nopriv disk access records on an HP-UX host. o If all devices on which simple or nopriv disks are present are not automatically configurable by VxVM. For example, third-party drivers export devices that are not automatically configured by VxVM. VxVM objects on simple/nopriv disks created from such disks are not affected by switching to the enclosure based naming scheme. The vxdarestore command does not handle the following cases: o If the enclosure-based naming scheme is in use and the vxdmpadm command is used to change the name of an enclosure, the disk access names of all devices in that enclosure are also changed. As a result, any persistent simple/nopriv disks in the enclosure are put into the "error" state, and VxVM objects configured on those disks fail. o If the enclosure-based naming scheme is in use and the system is rebooted after making hardware configuration changes to the host. This may change the disk access names and cause some persistent simple/nopriv disks to be put into the "error" state. o If the enclosure-based naming scheme is in use, the device discovery layer claims some disks under the JBOD category, and the vxdd- ladm rmjbod command is used to remove remove support for the JBOD category for disks from a particular vendor. As a result of the consequent name change, disks with persistent disk access records are put into the "error" state, and VxVM objects configured on those disks fail. EXIT CODES
A zero exit status is returned if the operation is successful or if no actions were necessary. An exit status of 1 is returned if vxdare- store is run while vxconfigd is using the c#t#d# naming scheme. An exit status of 2 is returned if vxconfigd is not running. SEE ALSO
vxconfigd(1M), vxdg(1M), vxdisk(1M), vxdiskadm(1M), vxdmpadm(1M), vxintro(1M), vxreattach(1M), vxrecover(1M) VxVM 5.0.31.1 24 Mar 2008 vxdarestore(1M)
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