Hello Recently I am learning about the Btrfs. My requirement is to construct a cross-device btrfs volume consisting of a single SSD and many (much larger) HDDs. The tricky part is that I want the SSD to be dedicated to metadata since SSDs are much faster (and more expensive) than HDDs and metadata are much more frequently queried than data. And I do not want ordinary data to use up the precious space on the SSD in case there would be no room for metadata. I have figured out that I can keep the metadata stored inside the first device by applying the "-m single" option to mkfs.btrfs. However, I do not know how to require that data should NOT be allocated on the SSD. I read the mkfs code. It appears to me that it can be worked around by excluding the SSD from the "raid group" for ordinary data. A recent patch have been proposed to move "hot" (according to statistics) data to SSDs in order to improve performance. My case is similar, but I know in advance that metadata are hotter. I am not very confident about what I have found, so I need some comment. Is it necessary to modify the Btrfs code? On the other hand, I have not figured out exactly how much metadata there would be with a given amount of data. How significant will it be to exclude ordinary data from the SSD? Kunshan Wang -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
