No, Unraid /does/ offer real time raid. The "sync" command you are talking about is related to the cache drives. You can setup a pool of SSD drives and tell different "shares" (think a folder than can span 1 more drives based on rules you set) to use that pool (BTRFS raid-1-type pool, so 2x1TB SSDs = 1TB cache pool). The "sync" (called "mover" in unraid terms) is a process that runs on a schedule you set (most people set it to run at night) that will move data to/from your cache drives to/from your raid array. The mover will not move files that still have an open handle on them (ie. you are actively using the file).
You can specify for each "share" if it should be No/Yes/Only/Prefer, explanation of each below (Pulled from my UnRaid install):
<Start copy>
* "No" prohibits new files and subdirectories from being written onto the Cache disk/pool.
* "Yes" indicates that all new files and subdirectories should be written to the Cache disk/pool, provided enough free space exists on the Cache disk/pool. If there is insufficient space on the Cache disk/pool, then new files and directories are created on the array. When the mover is invoked, files and subdirectories are transferred off the Cache disk/pool and onto the array.
* "Only" indicates that all new files and subdirectories must be written to the Cache disk/pool. If there is insufficient free space on the Cache disk/pool, create operations will fail with out of space status.
* "Prefer" indicates that all new files and subdirectories should be written to the Cache disk/pool, provided enough free space exists on the Cache disk/pool. If there is insufficient space on the Cache disk/pool, then new files and directories are created on the array. When the mover is invoked, files and subdirectories are transferred off the array and onto Cache disk/pool.
<End copy>
For me I just keep all my docker containers/VMs/docker config on my cache pool ("Only"). Everything else goes directly to the array ("No"). I do this because I gain nothing from having a new TV show sit on the cache for <24 hours. I am rarely going to watch something that fast and so it just fills up the cache drive for no good reason. Yes the array is slower to write to but I don't really mind that.
I am able to lose 1 cache drive (I have 2) and/or 1 data/parity drive without experiencing data loss. I currently have 1 SSD that is showing disconnected and 1 disk drive that is failing yet my system is chugging along without issue. Note: This is not my plan long-term, I think I bumped the sata cable for the SSD (when replacing a different drive) so I just need to open the case and as for the data drive I've got another one ready to go but I just haven't had the time in the past 2 days. Unraid is really great to work with IMHO.
You can specify for each "share" if it should be No/Yes/Only/Prefer, explanation of each below (Pulled from my UnRaid install):
<Start copy>
* "No" prohibits new files and subdirectories from being written onto the Cache disk/pool.
* "Yes" indicates that all new files and subdirectories should be written to the Cache disk/pool, provided enough free space exists on the Cache disk/pool. If there is insufficient space on the Cache disk/pool, then new files and directories are created on the array. When the mover is invoked, files and subdirectories are transferred off the Cache disk/pool and onto the array.
* "Only" indicates that all new files and subdirectories must be written to the Cache disk/pool. If there is insufficient free space on the Cache disk/pool, create operations will fail with out of space status.
* "Prefer" indicates that all new files and subdirectories should be written to the Cache disk/pool, provided enough free space exists on the Cache disk/pool. If there is insufficient space on the Cache disk/pool, then new files and directories are created on the array. When the mover is invoked, files and subdirectories are transferred off the array and onto Cache disk/pool.
<End copy>
For me I just keep all my docker containers/VMs/docker config on my cache pool ("Only"). Everything else goes directly to the array ("No"). I do this because I gain nothing from having a new TV show sit on the cache for <24 hours. I am rarely going to watch something that fast and so it just fills up the cache drive for no good reason. Yes the array is slower to write to but I don't really mind that.
I am able to lose 1 cache drive (I have 2) and/or 1 data/parity drive without experiencing data loss. I currently have 1 SSD that is showing disconnected and 1 disk drive that is failing yet my system is chugging along without issue. Note: This is not my plan long-term, I think I bumped the sata cable for the SSD (when replacing a different drive) so I just need to open the case and as for the data drive I've got another one ready to go but I just haven't had the time in the past 2 days. Unraid is really great to work with IMHO.