It depends on your network and bandwidth. You can monitor the CachePercentDirty CW metric to get an idea.
If the CachePercentDirty metric is 80 or greater, your file gateway is writing data faster to disk than it can upload the data to Amazon S3. Consider increasing the bandwidth for upload from your file gateway, adding one or more cache disks, or slowing down client writes.
If the CachePercentDirty metric is low, check the IoWaitPercent metric. If IoWaitPercent is greater than 10, your file gateway might be bottlenecked by the speed of the local cache disk. We recommend local solid state drive (SSD) disks for your cache, preferably NVM Express (NVMe). If such disks aren't available, try using multiple cache disks from separate physical disks for a performance improvement.***
You also should be aware of users writing/changing files on the FSx file system directly, and those changes being available on the gateway. This is the "Automated cache refresh settings" option. See: (THis is for Storage Gateway but the same concept applies for FSx)
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