Sent from my iPhone
> On May 25, 2023, at 7:37 PM, Gavin gao <gaozhang...@gmail.com> wrote:
>
> In a typical bookkeeper deployment, SSD disks are used to store Journal log
> data, while HDD disks are used to store Ledger data.
What is used is a deployment choice. I know that when OMB is run locally
attached SSDs are used for both.
I do agree that the choice of SSD and HDD disks can impact Bookkeeper
performance. Increasing IOPs and throughput will impact performance
significantly. For example in AWS a default gp3 attached disk will have large
latencies, but upping performance will give performance maybe 4x slower than an
SSD locally attached.
> Data writes are
> initially stored in memory and then asynchronously flushed to the HDD disk
> in the background. However, due to memory limitations, the amount of data
> that can be cached is restricted. Consequently, requests for historical
> data ultimately rely on the HDD disk, which becomes a bottleneck for the
> entire Bookkeeper cluster. Moreover, during data recovery processes
> following node failures, a substantial amount of historical data needs to
> be read from the HDD disk, leading to the disk's I/O utilization reaching
> maximum capacity and resulting in significant read request delays or
> failures.
>
> To address these challenges, a new architecture is proposed: the
> introduction of a disk cache between the memory cache and the HDD disk,
> utilizing an SSD disk as an intermediary medium to significantly extend
> data caching duration. The data flow is as follows: journal -> write cache
> -> SSD cache -> HDD disk. The SSD disk cache functions as a regular
> LedgerStorage layer and is compatible with all existing LedgerStorage
> implementations.
A different way to look at this is to consider the cold layer as being optional
and within HDD or even S3. In S3 you could have advantages with recovery into
different AZs. You could also significantly improve replay.
> The following outlines the process:
>
> 1. Data eviction from the disk cache to the Ledger data disk occurs on a
> per-log file basis.
> 2. A new configuration parameter, diskCacheRetentionTime, is added to
> set the duration for which hot data is retained. Files with write
> timestamps older than the retention time will be evicted to the Ledger data
> disk.
If you can adjust this to use a recent use approach then very long ledger can
be easily read with predictively moving ledgers from cold to hot.
> 3. A new configuration parameter, diskCacheThreshold, is added. If the
> disk cache utilization exceeds the threshold, the eviction process is
> accelerated. Data is evicted to the Ledger data disk based on the order of
> file writes until the disk space recovers above the threshold.
> 4. A new thread, ColdStorageArchiveThread, is introduced to periodically
> evict data from the disk cache to the Ledger data disk.
Anotger thread is also needed - ColdStorageRetrievalThread.
Just some thoughts.
Best,
Dave
