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EqualLogic Storage Pools Failure Potential
Posted on 2012-12-20
As anyone familiar with EquaLogic knows, a pool can have multiple members, but a member cannot participate in more than one pool. At least that is my understanding, please correct if I am wrong.
The next assumption I have is that given two identical members set at the same RAID level in a pool, when creating a volume in that pool the volume will eventually span the two members? Is that correct?
If a volume does span the members in a pool and one of the members completely fails (power power supplies in one member looses power for example), then the volume will essentially be offline, is that correct? Yes, there is a very slim chance that such a power event would occur in a properly designed and implemented environment, but I'm looking at the theoretical here.
What does EqualLogic do to prevent issues with the volume when such an event occurs?
The next assumption I have is that given two identical members set at the same RAID level in a pool, when creating a volume in that pool the volume will eventually span the two members? Is that correct?
If a volume does span the members in a pool and one of the members completely fails (power power supplies in one member looses power for example), then the volume will essentially be offline, is that correct? Yes, there is a very slim chance that such a power event would occur in a properly designed and implemented environment, but I'm looking at the theoretical here.
What does EqualLogic do to prevent issues with the volume when such an event occurs?
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You are correct that a pool can have multiple members and that every member is part of exactly one pool.
Volumes may span multiple members. With performance based load balancing introduced in the 5.x code, a single pool can have members of different drive and RAID types. the volumes in the pool can then span up to three (I believe) members.
If a member goes offline for whatever reason, then every volume that is on that member in whole or in part will go offline. To prevent such an occurance members have redundant controllers, power supplies, fans, and network connections. The backbone is passive. if there was to be a power failure that you didn't avoid by putting each power supply on a separate UPS, the controllers can store the write cache with either battery backup or store the cache in flash, depending on the model. As long as you power the member back on before the 3 day cache battery on older models runs out, you shouldn't lose any writes to the member.
Long story short, EqualLogic tries to keep your volumes available by keeping the members available. It is up to you to provide redundant power and network switches.
Volumes may span multiple members. With performance based load balancing introduced in the 5.x code, a single pool can have members of different drive and RAID types. the volumes in the pool can then span up to three (I believe) members.
If a member goes offline for whatever reason, then every volume that is on that member in whole or in part will go offline. To prevent such an occurance members have redundant controllers, power supplies, fans, and network connections. The backbone is passive. if there was to be a power failure that you didn't avoid by putting each power supply on a separate UPS, the controllers can store the write cache with either battery backup or store the cache in flash, depending on the model. As long as you power the member back on before the 3 day cache battery on older models runs out, you shouldn't lose any writes to the member.
Long story short, EqualLogic tries to keep your volumes available by keeping the members available. It is up to you to provide redundant power and network switches.
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