cloud: update docs for BYOC single AZ support

tidb-cloud/architecture-concepts.md

@@ -104,6 +104,8 @@ BYOC provides the following features:
 
 To meet different workload requirements, TiDB Cloud BYOC supports both Single-AZ and Multi-AZ deployments, enabling you to choose between lower-latency zonal deployment and higher-resilience deployment across multiple availability zones.
 
+You can configure high availability mode for each resource pool as **Zonal** or **Regional** when you [create the pool](/tidb-cloud/byoc/create-resource-pool-byoc.md). A zonal resource pool is deployed in a single availability zone, while a regional resource pool is deployed across multiple availability zones. TiDB Cloud determines the availability zone placement based on the BYOC region configuration and available cloud resources. TiDB Cloud BYOC instances created or restored in a resource pool inherit the high availability mode of the pool. After a resource pool is created, you cannot change its high availability mode or availability zone placement.
+
 ![TiDB Cloud BYOC Architecture](/media/tidb-cloud/byoc-architecture.png)
 
 A TiDB Cloud BYOC deployment includes the following major components:

tidb-cloud/byoc/byoc-initialize-service.md

@@ -11,7 +11,11 @@
 
 You can now provision resource pools and TiDB instances directly via the TiDB Cloud console.
 
-1. Create a resource pool. Log in to the [TiDB Cloud console](https://tidbcloud.com/) and follow the instructions in [Create a Resource Pool](/tidb-cloud/byoc/create-resource-pool-byoc.md) to create a resource pool in the target cloud provider and region.
+1. Log in to the [TiDB Cloud console](https://tidbcloud.com/), and then follow the instructions in [Create a Resource Pool](/tidb-cloud/byoc/create-resource-pool-byoc.md) to create a resource pool in the target cloud provider and region. 
+
+    >**Note:**
+    >
+    > When creating the resource pool, select **Zonal** or **Regional** high availability based on your workload requirements. {{{ .byoc }}} instances created or restored in the resource pool inherit the high availability mode of the pool.
 
 2. After the resource pool becomes **Active**, follow the instructions in [Create a {{{ .byoc }}} Instance](/tidb-cloud/byoc/create-tidb-instance-byoc.md) to create a new instance in the resource pool.
 
@@ -34,7 +38,9 @@
 
     Once the access keys are configured, you can initiate the restore job from the TiDB Cloud console.
 
-    During restore, select an active resource pool in the target cloud provider and region.
+    During restore, select an active resource pool with sufficient vCPU capacity in the target cloud provider and region. The restored instance inherits the high availability mode of the selected resource pool.
+
+    If the selected resource pool has reached its Pool vCPU Limit, increase or turn off the limit, or select another resource pool before continuing. This is because restoring to a new {{{ .byoc }}} instance might cause the total provisioned vCPUs to exceed the Pool vCPU Limit and affect the performance of all {{{ .byoc }}} instances in the same pool.
 
     For step-by-step restoration procedures, see [Restore backups from cloud storage](/tidb-cloud/premium/backup-and-restore-premium.md#restore-backups-from-cloud-storage).
 

tidb-cloud/byoc/byoc-prepare-environment-aws.md

@@ -37,9 +37,9 @@ TiDB is a distributed database that requires specific infrastructure for high av
 
     - **Option B: POC / Cost Optimization (Single-AZ).** Select exactly **1 AZ**. For example, `us-west-2a`.
 
-        > **Note:**
-        >
-        > Multi-AZ deployment functionality is disabled for this configuration.
+The availability zones prepared for the BYOC region determine where TiDB Cloud places resource pools. When creating a resource pool, you select either **Zonal** or **Regional** high availability. A zonal resource pool is placed in one availability zone. A regional resource pool requires at least three eligible availability zones in the region.
+
+If a BYOC region is initially prepared with a single availability zone, TiDB Cloud uses that availability zone for zonal resource pools. You can create a regional resource pool only when two additional eligible availability zones are available. If you plan to use regional resource pools, confirm during environment preparation that the region provides at least three eligible availability zones.
 
 ## Step 4. Create a hosted zone for TiDB
 

tidb-cloud/byoc/create-resource-pool-byoc.md

@@ -48,7 +48,20 @@ To create a resource pool, take the following steps:
 
     If you set a Pool vCPU Limit, make sure that the total vCPUs of the initial TiKV and TiFlash resources do not exceed 50% of the Pool vCPU Limit. This helps reserve capacity for future instance creation and automatic scale-out.
 
-6. In the **Pool CIDR** area, review or enter the CIDR range for the resource pool.
+6. In the **High Availability** area, select a high availability mode for the resource pool.
+
+    - **Zonal**: deploys the resource pool in a single availability zone for lower network latency.
+    - **Regional**: deploys the resource pool across multiple availability zones for higher availability and resilience to outages at the availability zone level.
+
+>**Note:**
+>
+>  - You must select a high availability mode before creating the resource pool.
+>  - If you select the **Regional** mode, make sure that the region for this pool contains at least three eligible availability zones. If this requirement is not met, the process of creating the regional resource pool might get stuck.
+> - TiDB Cloud determines the availability zones based on the [BYOC region configuration](/tidb-cloud/byoc/byoc-prepare-environment-aws.md#step-3-select-region-and-availability-zones-azs) and available cloud resources.
+
+    After the resource pool is created, you cannot change its high availability mode or availability zone placement.
+
+7. In the **Pool CIDR** area, review or enter the CIDR range for the resource pool.
 
     The resource pool CIDR is used as the network address range for resources in the resource pool and cannot be modified after the resource pool is created.
 
@@ -61,10 +74,6 @@ To create a resource pool, take the following steps:
 
     TiDB Cloud validates CIDR overlap with known TiDB Cloud managed network ranges in the target region. TiDB Cloud cannot detect overlap with your application VPCs, on-premises networks, or VPN networks. You must verify these customer-managed network ranges before creating the resource pool.
 
-7. In the **High Availability** area, review the high availability mode.
-
-    The high availability mode is inherited from the BYOC deployment configuration of the selected region. You cannot change it when creating a Resource Pool.
-
 8. (Optional) In the **AWS Resource Tags** area, add tags.
 
     Tags are applied to all taggable AWS resources within the resource pool.

tidb-cloud/byoc/create-tidb-instance-byoc.md

@@ -49,9 +49,14 @@
 
     Only resource pools that match the selected cloud provider and region and are in the **Active** status are displayed. If no suitable resource pool is available and you are an `Organization Owner`, click **Create Resource Pool** to create one. After the resource pool becomes active, it is automatically selected for the new instance.
 
+    Make sure to select a resource pool with sufficient vCPU capacity. Otherwise, the following issues can occur.
+
+    - If the selected resource pool has a Pool vCPU Limit and its current provisioned vCPU is greater than or equal to the limit, TiDB Cloud displays a warning, and you cannot create the {{{ .byoc }}} instance in that resource pool. To continue, go to the **Resource Pool** details page of the pool to increase or turn off the Pool vCPU Limit, or select another resource pool.
+   - Even if the current provisioned vCPUs are below the Pool vCPU Limit, creating a new instance might cause the total provisioned vCPUs to exceed the limit. This might restrict resource scaling and affect the performance of all instances in the resource pool. If necessary, increase or turn off the Pool vCPU Limit, or select another resource pool.
+
 7. In the **Capacity** area, set the maximum number of Request Capacity Units (RCUs) for your instance. RCUs represent the compute resources provisioned for your workload. TiDB Cloud automatically scales your instance within this range based on demand.
 
-8. For {{{ .byoc }}} instances, the high availability mode is inherited from the selected resource pool. If the resource pool uses regional high availability, the instance uses regional high availability. If the resource pool uses zonal high availability, the instance uses zonal high availability. You cannot configure the high availability mode when you create a BYOC instance. For more information, see [High Availability](/tidb-cloud/serverless-high-availability.md).
+8. For {{{ .byoc }}} instances, the high availability mode is inherited from the selected resource pool. If the resource pool uses regional high availability, the instance uses regional high availability. If the resource pool uses zonal high availability, the instance uses zonal high availability. You cannot configure the high availability mode when you create a BYOC instance. For more information, see [TiDB Cloud BYOC architecture](/tidb-cloud/architecture-concepts.md#tidb-cloud-byoc).
 
 9. Click **Create**.
 

tidb-cloud/byoc/manage-resource-pools-byoc.md

@@ -43,6 +43,7 @@ To view resource pool details, click the name of a resource pool on the **Resour
 
 The resource pool details page includes the following information:
 
+- **High Availability**: shows whether the resource pool uses zonal or regional high availability. This value is read-only. You cannot change the high availability mode or availability zone placement after the resource pool is created.
 - **Pool vCPU Usage**: shows the current provisioned TiDB vCPU and additional vCPU provision.
 - **Instances in This Pool**: lists the BYOC instances that run in this resource pool.
 - **Metrics**: shows vCPU usage and physical storage usage trends.
@@ -79,6 +80,12 @@ To update the Pool vCPU Limit, take the following steps:
 
 When you set a new Pool vCPU Limit, the value must be greater than or equal to `100` vCPU and the current provisioned TiDB vCPU. Otherwise, the update is rejected.
 
+> **Note:**
+>
+> Even when the current provisioned vCPU is below the Pool vCPU Limit, creating or restoring an instance might cause the total provisioned vCPU to exceed the limit. This might restrict resource scaling and degrade the performance of all instances in the resource pool. Before creating or restoring an instance, make sure that the resource pool has sufficient vCPU capacity.
+
+If the current provisioned vCPU of a resource pool is greater than or equal to its Pool vCPU Limit, you cannot create or restore an instance in that resource pool. To create or restore an instance, increase or turn off the Pool vCPU Limit, or select another resource pool.
+
 If the resource pool is in the **Modifying** status, **Update vCPU Limit** is disabled.
 
 ## Update capacity

tidb-cloud/byoc/multi-region-deployment.md

@@ -121,7 +121,7 @@ Use `tidbcloud-byoc-update.sh` to update the existing CloudFormation stacks. The
 Before running the update script:
 
 1. Confirm the AWS Regions to be added.
-2. Select the Availability Zones for the new regions.
+2. Select the Availability Zones for the new regions. A regional resource pool requires at least three eligible availability zones. If you plan to create regional resource pools in a new region, select at least three eligible availability zones for that region.
 3. Plan the O11Y CIDR for each new region and the resource pool CIDRs for the resource pools you plan to create. Different regions can use the same O11Y CIDR. However, if you use metric integration to connect Grafana to multiple regions, use non-overlapping O11Y CIDRs for those regions.
 4. Confirm whether each new region will:
     * share the existing PCA and TiDB hosted zone, or

tidb-cloud/premium/backup-and-restore-premium.md

@@ -242,7 +242,13 @@ To restore your data to a new {{{ .premium }}}<CustomContent plan="byoc"> or {{{
 
     <CustomContent plan="byoc">
 
-    For {{{ .byoc }}}, select an active resource pool in the same cloud provider and region as the backup. If no suitable resource pool is available, an `Organization Owner` can create one before restoring the instance. Other roles cannot create resource pools. For more information, see [Create a Resource Pool](/tidb-cloud/byoc/create-resource-pool-byoc.md).
+    For {{{ .byoc }}}, select an active resource pool in the same cloud provider and region as the backup. The restored instance inherits the high availability mode of the selected resource pool. You can restore the same backup to a Zonal or Regional resource pool. If no suitable resource pool is available, an `Organization Owner` can create one before restoring the instance. Other roles cannot create resource pools. For more information, see [Create a Resource Pool](/tidb-cloud/byoc/create-resource-pool-byoc.md).
+
+    If the selected resource pool has a Pool vCPU Limit and its current provisioned vCPU is greater than or equal to the limit, TiDB Cloud displays a warning and you cannot restore the instance to that resource pool. To continue, go to the Resource Pool details page to increase or turn off the Pool vCPU Limit, or select another resource pool.
+
+    > **Note:**
+    >
+    > Even if the current provisioned vCPU is below the Pool vCPU Limit, restoring an instance might cause the total provisioned vCPU to exceed the limit. This might constrain resource scaling and degrade the performance of all instances in the resource pool. Before restoring the instance, make sure that the resource pool has sufficient vCPU capacity. If necessary, increase or turn off the Pool vCPU Limit, or select another resource pool.
 
     </CustomContent>
 
@@ -366,7 +372,13 @@ To restore backups from cloud storage, do the following:
 
     <CustomContent plan="byoc">
 
-    For {{{ .byoc }}}, select an active resource pool that matches the target cloud provider and region. If no suitable resource pool is available, an `Organization Owner` can create one before restoring the instance. Other roles cannot create resource pools. For more information, see [Create a Resource Pool](/tidb-cloud/byoc/create-resource-pool-byoc.md).
+    For {{{ .byoc }}}, select an active resource pool that matches the target cloud provider and region. The restored instance inherits the high availability mode of the selected resource pool. You can restore the same backup to a Zonal or Regional resource pool. If no suitable resource pool is available, an `Organization Owner` can create one before restoring the instance. Other roles cannot create resource pools. For more information, see [Create a Resource Pool](/tidb-cloud/byoc/create-resource-pool-byoc.md).
+
+    If the selected resource pool has a Pool vCPU Limit and its current provisioned vCPU is greater than or equal to the limit, TiDB Cloud displays a warning and you cannot restore the instance to that resource pool. To continue, go to the Resource Pool details page to increase or turn off the Pool vCPU Limit, or select another resource pool.
+
+    > **Note:**
+    >
+    > Even if the current provisioned vCPU is below the Pool vCPU Limit, restoring an instance might cause the total provisioned vCPU to exceed the limit. This might restrict resource scaling and affect the performance of all instances in the resource pool. Before restoring the instance, make sure that the resource pool has sufficient vCPU capacity. If necessary, increase or turn off the Pool vCPU Limit, or select another resource pool.
 
     </CustomContent>