Web-server – Web server on SAN vs SSD

We have an EVA6100 so I've done tests on similar hardware. The 8000 is, IIRC, an older system than the 6100.

First off, your writes to the DB volume and Log volume are very likely contending for writes as it sounds like they're on the same Disk Group inside the EVA. A disk group is a grouping of disks that you create LUNs on, and each LUN is a disk presented to one (or more) servers. It is not at all uncommon to house many, many LUNs in a single disk-group. If you only have one disk-group, then the benefits of separating your log I/O from your DB I/O are somewhat reduced.

Second, if that DG is running into true I/O contention, then it'll degrade performance. This is something that's very hard to check from a consumer point of view, it's the kind of thing the SAN maintainers need to be aware of. Ask them if they know what the average controller CPU load is, as that'll significantly affect your write performance.

Third, know what your I/O mix really is an benchmark for that, not just file-copy speed. If your DB volume I/O is 80/20 read/write, then you really want to check read-path throughput. If it's the other way around, then your write performance is the metric you really need to check.

The key to a good VMWare storage platform is understanding what kind of load VMWare generates.

• First, since you host a lot of servers, the workload is typically random. There are many IO streams going at the same time, and not many of them can be successfully pre-cached.
• Second, it's variable. During normal operations, you may see 70% random reads, however the instant you decide to move a VM to a new datastore or something, you'll see a massive 60GB sequential write. If you're not careful about architecture, this can cripple your storage's ability to handle normal IO.
• Third, a small portion of your environment will usually generate a large portion of the storage workload.

The best way to approach building storage for a VMWare platform is to start with the fundamentals.

• You need the ability to service a large random read workload, which means smaller faster drives, as well as possibly SSD. Most modern storage systems allow you to move data around automatically depending on how it's accessed. If you are going to use SSD, you want to ensure this is how you use it. It should be there as a way of gradually reducing hot-spots. Whether you use SSD or not, it's beneficial to be able to put all the work across all the drives, so something with a type of storage pooling would be beneficial.
• You need the ability to service intermittent large writes, which doesn't care as much about the spindle speed of the underlying drives, but does care about the controller stack's efficiency and the size of the cache. If you have mirrored caching (which is not optional unless you're willing to go back to backups whenever you have a controller failure), the bandwidth between the two caches used for mirroring will be your bottleneck for large sequential writes, usually. Ensure that whatever you get has a high speed controller (or cluster) interconnect for write caching. Do your best to get a high speed front end network with as many ports as you can get while remaining realistic on price. The key to good front end performance is to put your storage load across as many front end resources as possible.
• You can seriously reduce costs by having a tier for low priority storage, as well as thin provisioning. If your system isn't automatically migrating individual blocks to cheap large/slow drives (like nearline SAS or SATA with 7200 RPM and 2TB+ sizes), try to do it manually. Large slow drives are excellent targets for archives, backups, some file systems, and even servers with low usage.
• Insist that the storage is VAAI integrated so that VMWare can de-allocate unused parts of the VMs as well as the datastores.