Microsoft’s web-based email service Hotmail is expected to unveil a number of new features at a press event early in October.However, Microsoft decided to reveal it’s new method that it has adopted for handling the data storage on Hotmail.On the Windows Live blog, Microsoft’s Kristof Roomp explains its upcoming storage improvements that will be put in place later this year.
Roomp explains “Hotmail’s storage system supports over one billion mailboxes and hundreds of petabytes of data (one petabyte is a million gigabytes, or a million billion bytes). The system services hundreds of thousands of simultaneous transactions from across the world.”
Microsoft has been using a RAID set up for the Hotmail storage system for while. For those who are not familiar with the term, A RAID (Redundant Array of Inexpensive Disks) system basically links up two or more hard drives to one controller board. To the operating system, multiple drives under a RAID set up look like one big hard drive.
Roomp says that,”This kind of storage system works fine if just one of the drives fails but,they don’t help if the whole machine or the RAID controller runs into problems. For larger drives,it was observed that having completely independent copies (on hard drives not sharing the same machine or controller) was much more reliable than a significantly more expensive RAID configuration.”
The new system ensures that the copies of data reside on independent hard drives, controllers, and machines. This kind of system is nicknamed “JBOD,” which stands for “Just a Bunch Of Disks.” In a JBOD system, the hard drive controller almost completely gets out of the way, which means that the software must now worry about all the failures that the controller previously handled. These failures can range from firmware bugs on the hard drives themselves to issues such as “unrecoverable read errors” that previously were automatically fixed by the controllers. In addition, the software must now scrub the drives periodically to check the data for “bit rot” (i.e., data that has for some reason become unreadable or corrupt).
The software we developed for the JBOD system monitors the hard drives schedules repair actions, detects failures, and diagnoses repairs. This software consists of a number of “watchdogs” that constantly monitor for certain types of failures. If the watchdog detects the failure that it is looking for, it raises an alert, which automatically triggers a repair process. This repair process can range from rebooting a machine or restarting a process, to fixing data corruption or even involving a human if progress can’t be made.
A big advantage of managing the drives in software is that the system knows exactly how many good copies of an email message we have. In the case where it finds that there are too few copies, it can prioritize repair actions to avoid a potentially dangerous situation. In situations where repairs are taking too long, it is possible to move data to another location altogether. This is also possible in RAID in a limited fashion, but it requires that every RAID controller has an extra spare drive hooked up to it, which increases costs significantly.
In addition, Hotmail will also add solid state drives to handle other functions. SSDs are much faster than normal hard drives but are also much more expensive. Roomp says the new Hotmail storage system will use SSDs to handle features like “the list of messages in your inbox, read/unread status of your messages, conversation threading, mobile phone synchronization etc.” This kind of data normally takes most of a hard drives activity. With SSDs handling this data and normal hard drives handling the actual email storage Roomp says, ” … we are able to take advantage of the trend in larger and cheaper hard drives without making any sacrifices in the performance of our system.”
